natural resources assessment

Irrigation ManualModule 2

Natural Resources Assessment

Developed by

Andreas P. SAVVAand

Karen FRENKEN

Water Resources Development and Management Officers

FAO Sub-Regional Office for East and Southern Africa

In collaboration with

Samuel SUNGURO, Hydrologist Consultant

Lee TIRIVAMWE, National Irrigation Engineer, Zimbabwe

Harare, 2002

Irrigation manual

ii – Module 2

Module 2 – iii

Contents

List of figures xiList of tables xiiList of abbreviations xiii

1. INTRODUCTION 11.1. Definitions 11.2. Resources assessment 2

2. LAND TOPOGRAPHY AND TOPOGRAPHIC SURVEYS 32.1. Topographic survey methods 32.2. Survey equipment and material 4

2.2.1. Accessories for survey instruments 42.2.2. Additional material for setting out the area and marking the stations 82.2.3. Materials for recording the data 8

2.3. The theodolite 82.3.1. Components of a theodolite 82.3.2. Setting up a theodolite 102.3.3. Selecting benchmarks 112.3.4. Reading and recording 112.3.5. Reading the angles 122.3.6. Measuring horizontal angles 132.3.7. Measuring vertical angles 142.3.8. The stadia system 142.3.9. Calculating horizontal distances, vertical heights and reduced levels 152.3.10. Accuracy 18

2.4. The level 182.5. Tacheometric survey using the theodolite 18

2.5.1. The reconnaissance survey 192.5.2. Station marking 192.5.3. The actual survey 192.5.4. Data processing 202.5.5. Plotting 272.5.6. Checking the theodolite 312.5.7 General remarks 33

2.6 Grid surveys 332.6.1. Setting out the grid 332.6.2. Checking the level instrument 352.6.3. The actual grid survey 362.6.4. Plotting 422.6.5. Use of pegs during implementation 42

3. SOILS AND SOIL SURVEY 453.1. Field observations 45

3.1.1. Soil pit description 453.1.2. Augering 463.1.3. Soil sampling 463.1.4. Infiltration test 47

3.2. Laboratory analysis 47

3.2.1. Mechanical analysis 473.2.2. Water-holding capacity or total available moisture 48

3.3. Soil map and soil report 50

4. SURFACE WATER RESOURCES 534.1. Water yield levels 534.2. Rivers 534.3. Dams and reservoirs 54

4.3.1. Sedimentation 544.3.2. Dam yields 56

5. GROUNDWATER RESOURCES 635.1. Groundwater resources and the hydrologic cycle 635.2. Groundwater occurrence 64

5.2.1. Aquifers 645.2.2. Aquifer types 66

5.3. Groundwater resources development 675.3.1. Groundwater exploration 675.3.2. Water wells 685.3.3. Collector wells 705.3.4. Groundwater resources evaluation 71

5.4. Pumping tests 725.4.1. The principle 725.4.2. Definition of terms 735.4.3. Hydraulic properties of confined aquifers 745.4.4. Hydraulic properties of leaky and unconfined aquifers 755.4.5. Recovery tests 765.4.6. Slug tests 765.4.7. Well performance tests 77

5.5. Groundwater recharge 795.5.1. Water balance equations 795.5.2. Chloride mass balance technique 795.5.3. Groundwater level fluctuations 805.5.4. Environmental isotopes 80

5.6. Groundwater management 805.6.1. Groundwater models 805.6.2. Lowering groundwater levels 805.6.3. Conjunctive use of surface water and groundwater 805.6.4. Groundwater monitoring 80

REFERENCES 83

iv – Module 2

Module 2 – v

List of figures

1. A level instrument with accessories 4

2. Aluminium tripods used with levels and theodolites 4

3. Plumb bob and its use 5

4. Holding and placing a ranging rod 5

5. Setting out a straight line using ranging rods 6

6. Surveying staffs 6

7. A level plate and its use 6

8. Single prismatic square 7

9. Setting out a right angle using a single prismatic square 7

10. Setting out a perpendicular line using a double prismatic square 7

11. Wooden (timber) pegs 8

12. Essential features of a theodolite 8

13. The Wild T2 universal theodolite and its components 9

14. Turning the foot-screws 10

15. Levelling of the theodolite 10

16. Reading the angles, using a Wild T2 universal theodolite 12

17. Reading the angles, using a Sokkisha TM20H theodolite 13

18. Reading the angles, using a Zeiss THEO 020B theodolite 13

19. Angle projection 13

20. Principle of two axes 14

21. Vertical angle measurement 14

22. The stadia system 15

23. Calculating the horizontal distance 16

24. Calculating the vertical height 16

25. Calculating the reduced level 17

26. Tilting level 19

27. Automatic level 20

28. A closed traverse or polygon 21

29. Coordinates of point P 22

30. Bearings for part of a traverse 23

31. Traverse with read internal angles, using the 360° graduation 24

32. Part of a traverse and coordinates 25

33. Traverse of four stations and calculation of coordinates 26

34. Plotting with a protractor 30

35. Interpolation between two reduced level points using graph paper 30

36. Adjustment of an inclined vertical reticle line 31

37. Face left and face right readings 32

38. Horizontal adjustment of the vertical reticle line 32

39. Vertical adjustment of the horizontal reticle line 32

40. Adjusting the optical plummet 33

41. Setting out of grid points 34

42. Reading grid points with the use of a measuring tape 35

43. Error experienced when setting out grid lines 35

44. Checking the level instrument 36

45. Prinicple of staff reading 36

46. The field survey methodology 37

47. Elimination of error due to disparallelism 39

48. Contour map 42

49. USDA soil texture triangle 47

50. Soil texture class determination, using the USDA soil texture triangle 48

51. Two methods for generalizing soil texture classes 48

52. Typical pF curves for silty clay and loamy fine sand 51

53. Streamflow hydrographs 54

54. Sedimentation in a reservoir created by a dam 55

55. Trap efficiency 56

56. Yield curves for dams with storage ratios greater than 0.5 61

57. The hydrologic cycle 63

58. Systems representation of the hydrologic cycle 64

59. Development of deposits (unconsolidated sedimentary aquifer) in a flood plain 65

60. An idealized sandstone aquifer (consolidated sedimentary formation) 65

61. Schematic illustration of groundwater occurrence in carbonate rock with secondary permeability and enlarged fractures and bedding plane openings 65

62. Various types of aquifers 66

63. Schlumberger VES electrode configuration 68

64. Continuous slot wire wound screen in an unconsolidated formation 69

65. Collector well located near a surface water body 71

66. Drawdown in a pumped aquifer 72

67. Drawdown curves for aquifers with high or low transmissivity and storativity values 72

68. Drawdown versus time for 3 piezometers, located at 30, 90 and 215 metres from a pumping well 75

69. Various head losses in a pumped well 77

70. Hydrograph showing continuous groundwater level decline 81

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List of tables

1. Topographic survey instruments and materials 4

2. Example of recording 12

3. Layout of a spreadsheet for calculating coordinates, using 360° graduation 28

4. Calculation of station coordinates, using a spreadsheet (360° graduation) 29

5. Extract of Nabusenga irrigation scheme (Zimbabwe) field survey data from the survey notebook: recording using the rise and fall method 40

6. Extract of Nabusenga irrigation scheme (Zimbabwe) field survey data from the survey notebook: recording using the height of line of collimation method 41

7. Example of a soil pit description 46

8. Typical basic infiltration rate 47

9. Range of available moisture contents for different soil types 50

10. Available moisture for different soil types 50

11. Example of hydrological data from Gwayi catchment in Zimbabwe 58

12. Yield/live storage rations 59

13. Estimate of the water balance of the world 64

14. Optimum groundwater entrance velocity through a well screen 70

15. Guidelines to range of water level measurements in a pumping well and in piezometers 73

Module 2 – vii

List of abbreviations

A Area

AM Available Moisture

BM Benchmark

BS Back Station

CA Catchment Area

CV Coefficient of Variation

D Distance

D Bulk density

DC Dam Capacity

DS Dead Storage

E Error or misclosure

E Evaporation

EF Evaporation Factor

EI Evaporation Index

EPD Equivalent Pore Diameter

FC Field Capacity

FS Forward Station

H Height

HP Hewlett Packard

Hz Horizontal angle

I Current

IS Intermediate Station

K Hydraulic conductivity

K Constant

L Length

MAI Mean Annual Inflow

MAR Mean Annual Runoff

P Precipitation

PVC Polyethylene Vinyl Chloride

PWL Pumping Water Level

PWP Permanent Wilting Point

Q Discharge

R Rainfall

R Recharge

R Resistance

R Runoff

RL Reduced Level

RO Reference Object

S Staff intercept

SA Sediment Allowance

viii – Module 2

SC Sediment Concentration

SM Sediment Mass

SM Soil Moisture

SMT Soil Moisture Tension

SR Storage Ratio

SV Sediment Volume

SWL Static Water level

T Transmissivity

TBM Temporary Benchmark

U Live storage capacity of dam

UNEP United Nations Environment Programme

V Velocity

V Vertical angle

V Vertical height

V Volume

VES Vertical Electric Sounding

WA Water Availability

WHC Water Holding Capacity

WP Wilting Point

X X coordinate

Y Y coordinate

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Module 2: Natural resources assessment

Irrigation manual

x – Module 2

Module 2 – 1

Land is not regarded simply in terms of soil and topography,but encompasses features such as underlying superficialdeposits, climate and water resources, as well as the plantand animal communities that developed as a result of theinteraction of these physical conditions (FAO/UNEP,1999). The results of human activities, reflected by changesin vegetative cover or by structures, are also regarded asfeatures of the land. Changing one of the factors, such asland use, has potential impacts on other factors, such asflora and fauna, soils, surface water distribution andclimate. Changes in these factors can be readily explainedby ecosystem dynamics, and the importance of theirrelationships in planning and management of landresources has become increasingly evident.

1.1. Definitions

Land and land resources: Land and land resources referto a delineable area of the earth’s terrestrial surface,encompassing all attributes of the biosphere immediatelyabove or below its surface, including those of the near-surface climate, the soil and terrain forms, the surfacehydrology (including shallow lakes, rivers, marshes andswamps), the near-surface sedimentary layers andassociated groundwater and geohydrological reserve, theplant and animal populations, the human settlementpattern and physical results of past and present humanactivity, such as terracing, water storage or drainagestructures, roads, buildings, etc. (FAO/UNEP, 1997).

Soil: Soil is a three-dimensional body, occupying theuppermost part of the earth’s crust, having propertiesthat differ from the underlying rock material as a result ofinteractions between climate, living organisms (includinghuman beings), parent material and relief over periods oftime. Distinctions are made between ‘soils’ in terms ofdifferences in internal characteristics and/or in terms of thegradient, slope-complexity, micro-topography andstoniness and rockiness of surface. ‘Soil’ is a narrowerconcept than ‘land’, soil is one of the attributes of land(Euroconsult, 1989).

Landform: Landform refers to any physical, recognizableform or feature on the earth’s surface, having acharacteristic shape, and produced by natural causes; it

includes major forms such as a plain, plateau, or mountain,and minor forms such as a hill, valley, slope, esker, or dune.Taken together, the landforms make up the surfaceconfiguration of the earth.

Landscape: Landscape is a distinct association oflandforms, as operated on by geological processes (exo- orendogenic), that can be seen in a single view.

Topography: Topography encompasses the relief andcontours of a land surface.

Land cover: Land cover is the observed (bio)physicalcover on the earth’s surface (Di Gregorio and Jansen,1998).

Land use: Land use is characterized by the arrangements,activities and inputs by people to produce, change ormaintain a certain land cover type (Di Gregorio and Jansen,1998). Land use defined in this way establishes a direct linkbetween land cover and the actions of people in theirenvironment.

Land surveying: Land surveying deals with themeasurements of land and its physical features accurately,and the recording of these features on a map. It isconcerned with three main activities, namely measurementof length, levelling, and angular measurements. Landsurveying comprises geodetic surveying, topographicsurveying, photogrammetry, cadastral surveying,hydrographic surveying and engineering surveying.

Land levelling: Land levelling is the process of measuringthe difference in elevation between two or more points.

Land evaluation and land classification: Landevaluation is the process whereby the suitability of landfor specific purposes, such as irrigated agriculture, isassessed (FAO, 1985a). Land evaluation involves theselection of suitable land and suitable cropping, and ofirrigation and management alternatives that arephysically and financially practicable and economicallyviable. The main product of land evaluation is a landclassification that indicates the suitability of various kindsof land for specific land uses, usually depicted on mapswith accompanying reports.

Chapter 1Introduction

1.2. Resources assessment

Land evaluation and classification for irrigation purposes isa multidisciplinary undertaking involving soils scientists,hydrologists, irrigation specialists, environmentalists,sociologists, extensionists, agricultural economists, etc. Foran in-depth study of land evaluation for irrigated agriculturethe reader is referred to FAO (1985a).

This module deals with the natural resources assessmentand particularly concentrates on topography andtopographic surveys (Chapter 2), soils and soil surveys(Chapter 3), surface water resources (Chapter 4) andgroundwater resources (Chapter 5).

Other aspects, like climate, environment, health and socio-economic aspects, economic and financial appraisal,irrigation engineering, are covered in other modules.

Irrigation manual

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Land topography is often a major factor in irrigationevaluation as it may influence the choice of irrigationmethod, drainage, the type of erosion, irrigation efficiency,costs of land development, size and shape of fields, labourrequirements, range of possible crops, etc.

Four aspects of topography that have a special bearing onirrigation suitability are slope, micro-relief, macro-reliefand position (FAO, 1985a):

� Slope: Slope may affect the following factors: intendedmethods of irrigation, erodibility and erosivity,cropping pattern, mechanization problems, exposureto wind, etc. Slope limitations vary greatly fromcountry to country. Critical limits for differentmethods of irrigation are given in Module 1 andModule 7.

� Micro-relief: This term applies to minor surfaceundulations and irregularities of the land surface.Estimates of grading and levelling requirements willdepend on whether surface, sprinkler or localizedirrigation techniques are used. The informationrequired for an assessment of land grading costsincludes: cut and fill, the total volume of earth moved,the depth of cut, distance of transport, soil conditionsand desired precision of the final grading and type ofequipment available. Details on land levelling are givenin Module 7. Topsoil depth and subsoil quality maylimit the amount of grading that is advisable, or greatlyincrease the cost if it is necessary to conserve and laterrespread the topsoil. Some subsoils are unproductive atfirst, but gradually rehabilitate with irrigation andfertilizer or organic matter applications.

� Macro-relief: Permanent topographic features whereslopes change frequently in gradient and direction mayinfluence the choice of irrigation method, field sizesand shape, and land development costs.

� Position in relation to command area and accessibility: Theelevation and distance of the water source often affectsthe ‘irrigable’ land area in gravity-fed schemes. Thearea commanded may be increased by pumping, or byconstructing tunnels, inverted siphons and otherstructures through natural or human-made barriers, orby reservoir construction. Topographic data are oftenused in evaluating the infrastructural alternatives and

their land development costs. Topographic data are alsorequired in the case of flood hazard and the design offlood protection measures and for the design of surfaceor subsurface drainage.

Irrigation designs require contour intervals that shouldnormally be not more than 0.5 m, and an appropriate mapscale is required. Very detailed topographic data arerequired for many irrigation structures, especially alongroutes of probable canals and drains.

2.1. Topographic survey methods

Topographic surveys aim at describing the land topography.A topographic survey should always be carried out for thepreparation of a contour map, which will serve as the basisfor the design of any irrigation scheme. Two widely usedmethods of topographic surveying are the tacheometricsurvey (Section 2.5) and the grid survey (Section 2.6).When dealing with small surface irrigation systems, withsmall differences in elevation, a grid survey isrecommended. For pressurized irrigation systems(sprinkler, drip, etc.), the tacheometric method is usuallyused. As a rule, grid surveys require more time in the fieldand less in calculations, while the time requirements for thetacheometric method are the opposite.

While formerly only analogue instruments were available fortacheometric surveys, at present digital instruments are morereadily available on the market. The main difference betweenan analogue and a digital instrument is the read-out method.Instead of an operator having to note down a reading from ameasuring staff, a digital instrument is able to take anautomatic reading using barcodes on the staff, which isdefinite for every segment, and calculate the correspondingheight. It is able to do this in just a few seconds and removesthe possibility of a reading error by the operator. Readings areautomatically recorded on a PC-card.

For training purposes, this Module will deal with theanalogue instruments and will explain the manualcalculations as well as some basic programmes that can beused for the calculations. The reason for this is that a personwho knows how to work with an analogue instrument will,in general, also be able to work with a digital instrument,while the inverse is not necessarily the case. Although someof the equipment explained in this Chapter is no longer

Module 2 – 3

Chapter 2Land topography and topographic surveys

produced, such as the Wild T2 theodolite, it is still availablein most government departments in the region and will bestill used in the future by irrigation engineers and surveyors.This is another reason for describing this type of equipmentin detail in this Chapter.

2.2. Survey equipment and material

The following equipment and materials are needed toconveniently carry out a topographic survey:

� The survey instrument (theodolite or level instrument)plus accessories (tripod, plump bob, ranging rods, staff,level plate, measuring tape, prismatic square)

� Equipment and materials for the reconnaissance surveyand for setting out the area, marking the stations andinstalling the benchmarks (plumb bob, ranging rods,measuring tape, prismatic square, steel and woodenpegs, cement, stone and sand, picks and shovels,buckets, white paint, hammer)

� Materials for recording the data (ruler, pencil, sharpener,notebook, preferably a pocket calculator as well)

Table 1 lists the survey equipment and materials and thesurvey method associated with each one of them.

Some of the equipment and materials are briefly describedin the following sections. The theodolite and the levelinstrument are described more in detail in Section 2.3 and2.4 respectively.

2.2.1. Accessories for survey instruments

Figure 1 gives an overview of accessories for a surveyinstrument.

Tripod

The tripod is the three-legged support on which the surveyinstrument is mounted (Figure 1 and 2). When setting up,the legs of the tripod are extended until the tripod head isroughly at eye level. Then the legs are spread so that the tips

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Figure 1

A level instrument with accessories

(Source: Eijkelkamp, undated)

Figure 2

Aluminium tripods used with levels and theodolites (Source: Leica, 1993)

Table 1

Topographic survey instruments and materials

Instrument/materials Survey method

Tacheometry Grid

Theodolite x

Level (automatic, dumpy,

tilting, etc.) x

Tripod x x

Plumb bob x

Ranging rods x x

Staff x x

Measuring tape x x

Prismatic square x

Steel and wooden pegs x x

Hammer x x

Cement, stone, sand x x

Picks and shovels x x

Bucket x x

White paint x x

Notebook, ruler, pencil,

sharpener, calculator x x

form a regular triangle on the ground, after which thetripod legs are firmly fixed into the ground.

Plumb bob

The plumb bob consists of a piece of metal (the bob)attached to a string (Figure 3). It is used to check whethera level instrument or prismatic square is centrally locatedabove a point, for example a peg, or to check whetherobjects are vertical (Figure 4). On a theodolite the opticalplummet replaces the plumb bob (Section 2.3).

Ranging rod

Ranging rods are straight round poles, usually 2, 2.5 or 3 mlong and made of wood or metal (Figure 1 and 4). They are

normally painted with alternate red and white bands of 0.5m length each. They are tipped with a pointed steel shoe toenable driving them into the ground. The correct way tohold a ranging rod is to keep it loosely between thumb andindex finger, about 10 cm above the soil (Figure 4a). Whenthe observer indicates that the ranging rod is in the rightposition, the person holding the rod loosens it. The sharpbottom point leaves a mark on the soil exactly where therod has to be placed. Once in place, it should be checked ifit is vertical, for example with a carpenter level or a plumb(Figure 4b). Ranging rods are used for sighting in straightlines, marking points, etc. Figure 5 shows how they areused to make a straight line. After fixing the position of tworods (A and B), by using a survey instrument for example,other rods (C and D) are used to continue setting out otherpoints on that straight line.

Module 2 – 5


Figure 3

Plumb bob and its use (Source: FAO, 1985b)

Figure 4

Holding and placing a ranging rod (Source: FAO, 1985b)

a) Holding a ranging rod b) Placing a ranging rod

The measuring staff

Measuring staffs used in topographic surveys are usuallybetween 3 and 5 m long. There are folding staffs, which canbe unfolded and folded into 1 m sections, and there aretelescopic staffs, consisting of parts that can slide over one

another to compress or elongate. Most modern designs aremanufactured in aluminium alloys. They have a centimetregraduation and readings from the staff can be estimated at1 mm. The upper 5 cm (5 x 1 cm) of the 10 cm intervalare connected by a vertical band to form an E-shape,natural or reversed (Figure 6). The graduations of the firstmetre length are coloured black on a white background,while the graduations of the second metre length arecoloured red on a white background (Figure 1). The twocolours are repeated alternately for the subsequent metres.

Level plate

A level plate is a small steel plate with sharp points at thebottom (Figure 7). It can be used to give a firm surface forthe measuring staff to ensure that the elevation does notchange when turning the staff during a change point.

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Figure 5

Setting out a straight line using ranging rods (Source: FAO, 1985b)

Figure 6

Surveying staffs (Source: Leica, 1993)

Figure 7

A level plate and its use (Source: Leica, 1993)

Measuring tape

Measuring tapes are made of steel, linen or syntheticmaterial. They are available in lengths of up to 100 m, withgraduation in centimetres and metres. Tapes are used tomeasure distances. It is important that measuring tapes arewiped clean before rewinding into their cases.

Prismatic square

A prismatic square consists of a metal frame with a handlein which one prism (single prismatic square) or two prisms(double prismatic square) are fitted. They are used to setout right angles and perpendicular lines. Figure 8 shows asingle prismatic square. With the single prismatic squareone can fix right angles (Figure 9). The two prisms of thedouble prismatic square make it possible to look, at thesame time, at a right angle to the left, a right angle to theright and straight ahead through openings above and belowthe prisms. This makes it possible to see the baseline and

Module 2 – 7


Figure 10

Setting out a perpendicular line with a double prismatic square (Source: FAO, 1985b)

Figure 9

Setting out a right angle with a

single prismatic square (Source:

FAO, 1985b)

Figure 8

Single prismatic square (Source: FAO, 1985b)

the perpendicular line at the same time, therefore noassistant is needed to check whether the operator isstanding on the baseline (Figure 10).

2.2.2. Additional material for setting out the areaand marking the stations

Pegs and hammer

Pegs are made of steel, timber, or straight tree branches.The length should be 40 to 60 cm and they should have asharp point to ease them being driven into the soil, with a

hammer if necessary (Figure 11). They are used forindicating points in the field that require more permanentor semi-permanent marking, for example the benchmarks(steel pegs) or points of a grid (wooden pegs).

Cement, concrete stone, sand, picks, shovels, paint

These materials are used to make benchmarks andpermanent points on the baseline (grid survey) and surveystations (tacheometric survey).

2.2.3. Materials for recording the data

Notebooks, rulers, pencils, sharpeners, pocket calculatorare used for the actual recording of data during the fieldsurvey.

2.3. The theodolite

2.3.1. Components of a theodolite

The theodolite is an instrument used to measure bothhorizontal and vertical angles. It consists of the followingmain parts: a fixed base with tribrach, a movable upper partand a telescope (Figure 12). It is one of the most importantinstruments used in survey work. Different types oftheodolites are available from different manufacturers, butthey all have basically the same components, as shown inFigure 12 and 13, and the main ones are described below.

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Figure 11

Wooden (timber) pegs

Figure 12

Essential features of a theodolite

(Source: Wild Heerbrugg, undated)

� The base, with the tribach, is fixed on the tripod withone clamping screw. The theodolite is centered overthe station by means of a plumb bob or a built-in opticalplummet. Rough levelling-up of the base is done withthe circular bubble, by means of three foot-screws (seeSection 2.3.2). The tribrach supports the remainder ofthe instrument. Many instruments have the facility fordetaching the upper part of the theodolite from thetribrach, which is useful when transporting thetheodolite.

� The lower plate carries the horizontal circle, which can berotated independently of the base.

� On the upper plate, or alidade, which is rotatable aboutthe vertical or standing axis, the two standards for thehorizontal or trunnion or tilting axis, bearing thetelescope (with the sighting axis) and the vertical circle, arefixed. The alidade also contains the reading (system)index of the horizontal circle.

� The plate level, or alidade tubular level, is used for moreaccurate levelling-up after the rough levelling-up hasbeen done using the circular bubble at the base.

� The upper plate and lower plate each have separate clampsand slow motion drives or screws. The upper plate screws

are milled and the lower plate screws are serrated. Ifthe lower plate is clamped and the upper plate is free,rotation in azimuth (horizontal) gives different readingson the horizontal scale. If the lower plate is free and theupper plate is clamped, rotation in azimuth retains thehorizontal scale reading, thus the horizontal circle rotates.

� The telescope, attached to the tilting axis, can be aimed inany direction in space, by means of rotations about itsstanding and tilting axes. Fine pointing to a particulartarget is achieved by using the clamps and the slowmotion screws.

� Also attached to the tilting axis is the circle readingmicroscope, the micrometer knob or screw, and the verticalcircle. The vertical circle usually has an index level so thatit can be oriented correctly, in relation to thehorizontal, before a vertical angle is read.

� The focusing sleeve or screw is attached to the main frameof the telescope just in front of the eyepiece.

� The side of the main telescope, viewed from theeyepiece, containing the vertical circle is called the face.

� When the main telescope is rotated about the tilting axisfrom one direction to face in the opposite direction, itis said that it has been transmitted.

Module 2 – 9


Figure 13

The Wild T2 universal theodolite and its components (Source: Wild Heerbrugg, undated)

Carrying handle

Detachable tribrach

Horizontal clamp

Vertical drive

Telescopic eyepiece

Vertical clamp

Optical sight

Optical plummet

Circular bubble

Micrometer knob

Horizontal drive

Plate level

Selector knob for Hz and V cicles

Reading microscope

Focussing sleeve

2.3.2. Setting up a theodolite

First of all, the tripod is set up over a station, normally a pegcontaining a steel rod or a nail at the centre. The legs areplaced at an equal distance from the peg and their heightadjusted to suit the surveyor. The tripod head should bemade as level as possible by eye.

After the tripod has been set up, the theodolite is carefullytaken out of its case, its exact position being noted to assistin replacement, and is securely attached to the tripod head.The theodolite should always be held by the standards, notby the telescope.

The theodolite is then centered roughly over the stationwith the optical plummet by shifting two legs of the tripod,leaving one on the ground. The tripod legs must then be

firmly pressed into the ground to avoid any furthermovement as the surveyor moves around it or when heavytraffic passes nearby. The machine is leveled roughly bymoving the tripod legs up or down until the bubble of thecircular level is centered. When doing fine levelling thefoot-screws (Figure 12) are rotated as shown in Figure 14.First, two foot-screws are turned simultaneously inopposite directions. After that, the third screw is turned.

The fine levelling of the machine is carried out as follows(Figure 15):

1. Looking from the top, the alidade is rotated until theplate level is located between two foot-screws, as inFigure 15a. These two foot-screws are turned until theplate level bubble is brought to the centre of its run.The levelling foot-screws are turned in oppositedirections simultaneously, as shown in Figure 14,remembering that the bubble will move in a directioncorresponding to the movement of the left thumb.

2. The alidade is turned through 90° clockwise (Figure15b) and the bubble centered, again using the thirdfoot-screw only.

3. The above operations are repeated until the bubble iscentered in both positions a and b.

4. The alidade is now turned until it is in a position 180°clockwise from position a, as in Figure 15c. Theposition of the bubble is noted.

5. The alidade is turned through a further 90° clockwise,as in Figure 15d, and the position of the bubble is againnoted.

6. If the bubble is still in the centre of its run for both step4 and 5 above, the theodolite is level and no furtheradjustment is needed. If the bubble is not central itshould be off-centre by the same amount in both step4 and 5. This may be, for example, 2 divisions to theleft.

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Figure 15

Levelling of the theodolite

Figure 14

Turning the foot-screws

7. To remove the error, the alidade is returned to itsinitial position (Figure 15a). Again using the two foot-screws located at each side of the plate level (seenfrom above), the bubble is placed in such a positionthat half the error is taken out, for example 1 divisionto the left.

8. The alidade is then turned through 90° clockwise as inFigure 15b and half the error is again taken out.

9. Step 7 and 8 are repeated until half the error is takenout for both positions.

10. The alidade is now slowly rotated through 360° and theplate level bubble should remain in the same position.

11. Loosen the clamping screw, which fixes the theodoliteonto the tripod, and center with the optical plummet.Check again if it is still level. If not, repeat the stepsstarting from 1.

Miscentering is often an important source of mistakes, sothe above steps should be carried out very carefully.

Next, parallax should be eliminated by accurately focusingthe cross hairs (lines) against a light background andthereafter focusing the instrument on a distant target.

2.3.3. Selecting benchmarks

All topographic surveys should start from a benchmark,either permanent or temporary. Permanent benchmarks(BM) are points of known elevation with reference to anational grid (and linked to the mean sea level). Temporarybenchmarks (TBM) can be used as reference points forsurveys. They are established as local reference points for aparticular survey and can either be related or unrelated tothe national grid of elevations. They should be cast inconcrete with dimensions of at least 30 cm x 30 cm x60 cm (length x width x depth). A pin, for example a steelrod of 10 mm diameter, is embedded in the centre of thebenchmark and is used to put the staff on for the actualreading. If present near the area to be surveyed, a BM withknown height can be used to give a height to the TBM. Ifthere is no BM in the vicinity, the TBM can be given anarbitrary value, for example 100.00 m. It is notrecommended to choose 0.00 m as value for the TBM inorder to avoid negative heights of field points.

2.3.4. Reading and recording

It is important to measure the height of instrument (fromthe red dot at the trunnion or tilting axis to the top of peg).This should be done first, as it is easy to forget about itafterwards when one has gone through a number ofreadings.

The base is fixed by locking the serrated screw with yellowdot while pointing north at 00°00'. Use the compass onthe side of the theodolite to locate North. If there is nocompass, point the telescope approximately to the North.

Unlock the alidade with the milled screw and shoot anumber of reference objects (RO), such as the corner of ahouse, tank, electricity pole, etc., and note down the angle.

Shoot the forward station or fore sight (FS) and note allreadings: the three hair (horizontal line) readings, thevertical angle and the horizontal angle. It is good practice toread the horizontal angle last, as this angle should remainthe same when moving the machine (see below).

After shooting all necessary points (a grid of 20 m x 20 mis appropriate for irrigation system designs), read back tothe RO and check for mistakes. Then shoot the FS again,carry the bearing as explained below, and continue thesurvey, starting with the station from where the machinewas moved and which is now the back station or back sight(BS).

Carrying the bearing comprises the following steps (see alsoSection 2.5.4):

a. Leave the alidade fixed to the lower plate (locked)

b. Unlock the base by opening the serrated screw withthe yellow dot, so that the theodolite can turn free onthe tribrach. The same horizontal angle noted whenviewing the FS can still be read.

c. Detach the theodolite from the tribrach and place itback in its case.

d. Pick up the tripod and move to the FS.

e. Center and level the theodolite above this new stationas explained earlier.

f. Transit the telescope (face right) and shoot the BS sothat the vertical hair is on the centre of the staff.

g. Lock the base and do the fine adjustment with the slowmotion serrated base-screw, until the vertical hair isexactly in the centre of the staff.

h. Transit the telescope, unlock the alidade (with themilled screw) and swing the alidade 180°. Point againat the BS with the vertical hair on the staff. Always tryto have the vertical hair coincide with the centre of thelower part of the staff, ensuring a more accuratepositioning of the peg.

i. Fix the alidade and fine adjust with the slow motionmilled screw.

Module 2 – 11


The reading of the horizontal angle should exactly read thehorizontal angle from the previous station to the presentstation plus 180° or minus 180°. Table 2 gives an exampleof the recording of the readings for two stations.

2.3.5. Reading the angles

Different theodolites can have different methods ofreadings. In addition to the method given for the Wild T2theodolite (which is the theodolite that has been describedas an example in Section 2.3.1), two other examples are alsogiven below.

Circle reading method of the Wild T2 UniversalTheodolite

The eyepiece of the reading microscope is turned until thecircle graduation lines (top window of Figure 16) are in

focus. The horizontal (Hz) or vertical (V) circle is thenselected using the selector knob. The Hz and V readingcircles are distinguished by the colour of the windows.Yellow is for the Hz circle and white is for the V circle. Themethod of reading is the same for both circles. Theestimation of reading is 1". Figure 16 shows the reading forthe 360° model (a) and for the 400g model (b).

Circle reading method of the Sokkisha TM20HTheodolite

Minutes (') and seconds (") are read by turning theadjustment screw until the single hair moves exactly inbetween the double hairs of the horizontal angle readingor the vertical angle reading. The graduation value of thereading scale is 10" and the estimation of the reading is5".

Irrigation manual

12 – Module 2

Table 2

Example of recording

Instrument Shooting Hair reading Vertical angle Horizontal angle Remarks

position at Lower Middle Upper

BM1

(height of P1 1.00 2.00 3.00 90.51.15 241.01.50 FS

instrument is Intermediate 1.00 1.50 2.00 90.13.10 200.11.20 field edge

135.5 cm) Intermediate 0.50 1.00 1.50 90.04.00 191.10.20 tree

. . . . . . .

. . . . . . .

. . . . . . .

P1 1.00 2.00 3.00 90.51.15 241.01.50(a) FS

P1

(height of BM1 1.50 2.50 3.50 89.09.45 61.01.50(b) BS

instrument is P2 1.00 1.90 2.80 89.10.20 250.40.30 FS

140.0 cm) Intermediate 0.40 0.80 1.20 89.00.10 80.25.00 Anthill

. . . . . . .

. . . . . . .

. . . . . . .

P2 1.00 1.90 2.80 89.10.20 250.40.30 FS

(a) should be equal to (b) minus (or plus) 180°

Figure 16

Reading of the angles, using a Wild T2 universal theodolite (Source: Wild Heerbrugg, undated)

a. Hz = 94°12'44" (360° graduation) b. Hz = 105,8224g (400g graduation)

Only one angle is read at a time. For example, in Figure 17only the horizontal angle is read. In order to make thevertical angle reading, the adjustment screws have to beused again to move the single hair of that angle to read inbetween the double hairs.

Circle reading method of the Zeiss THEO 020Btheodolite

Both the horizontal and vertical angles are readsimultaneously without adjustments (Figure 18). Thegraduation value of the reading is 1' and the estimation ofthe reading 10".

2.3.6. Measuring horizontal angles

An angle formed by the points APB will be indicated by theangle formed by the points A'PB' when projected on to amap (Figure 19).

All geodesy calculations are done in the horizontally-projected plane. Thus one is interested in the angle A'PB'.A theodolite is an instrument that can be used to measureangles in the horizontal plane.

As explained in Section 2.3.1, the theodolite has atelescope, rotating around the horizontal or tilting axis(called the second axis). The telescope and the second axiscan rotate together around the vertical or standing axis(called the first axis) (Figure 20).

Module 2 – 13


Figure 17

Reading of the angles, using a Sokkisha TM20H

theodolite

Hz = 327°29'10"

Figure 18

Reading of the angles, using a Zeiss THEO 020B theodolite

a. Hz = 217°06'20"

b. V = 93°03'00"

Figure 19

Angle projection

Furthermore, there is a horizontal plate with divisions indegrees, perpendicular to the first axis. A reading device orscale is connected to the first axis and rotates around it,together with the telescope and second axis, over thehorizontal plate.

With the theodolite located at point P (Figure 19), it willmake no difference if one shoots A or A'. Also, whenturning the telescope around the vertical axis to shoot B, itdoes not make a difference if one shoots B or B'. In otherwords, when shooting points A and B, the differencebetween the two readings on the horizontal plate is theangle between the two horizontal projections of thesepoints (angle at A' minus angle at B').

2.3.7. Measuring vertical angles

Besides measuring horizontal angles, theodolites are alsoequipped to measure vertical angles. Angles can be readfrom the vertical circle (Figure 21).

The vertical circle sits on the side of the telescope,perpendicular to the second axis and rotates with thetelescope around this axis. In nearly all theodolites onereads zenith angles, which means that pointingperpendicularly upwards is 0° and horizontally is 90°.

As explained in Section 2.3.6, all points read with thetheodolite are plotted on a map as the horizontal projectionof these points. If a vertical angle is involved when readinga point, the horizontally projected distance from thetheodolite to that point will not equal the distance of theline of sight or slope distance. The difference is larger whenthe vertical angle is close to 0° and smaller when the verticalangle is close to 90°.

In the particular case where, for example, a sprinklerirrigation system has to be implemented on sloping land(areas going up to 4-5% slope), the total length of the pipesto be ordered based on the distances calculated from thetopographic map will be less than when calculated on thereal slope distance. At the same time, friction losses will beslightly underestimated. However, by allowingcontingencies on the total bill of quantities and on the totaldynamic head of the system, this difference will be takencare of.

2.3.8. The stadia system

When looking through the telescope, three horizontal hairs(or lines) can be noticed (lower, middle and upper). Thelower and upper hairs are called stadia lines. Stadia linesintersect the image of the staff and define a fixed angle. Thedistance between the hairs is fixed and is called stadiainterval. When viewed through the telescope, the stadiahairs cover a certain length S on the staff. The value of S

Irrigation manual

14 – Module 2

Figure 20

Principle of two axes

Figure 21

Vertical angle measurement

depends on the horizontal distance D between theinstrument and the staff (Figure 22). In this case the staff isread with a horizontal line of sight.

2.3.9. Calculating horizontal distances, verticalheights and reduced levels

Horizontal distance

The horizontal distance D can be calculated from thefollowing equations, which are derived from Figure 22.

From the similar triangles, it follows:

Equation 1

d =

f

S i

and:

Equation 2

d = D - (f + c)

Combining Equation 1 and 2 results in the following:

D - (f + c) =

f

S i

or:

D = f

x S + (f + c)i

Withf

= stadia constant K, and f + c = constant C,i

the formula can be written as:

Equation 3

D = K x S + C

For most theodolites K = 100 and C = 0, resulting in:

Equation 4

D = 100 x S

The middle hair divides S into two equal distances, whichgives the possibility of verifying the readings. A difference of2 to 10 mm between the top and lower half intercept isacceptable, but more than 10 mm indicates an error thatneeds to be checked.

Obviously, multiple height differences between the staffposition and the instrument will make it impossible to readthe staff with a horizontal line of sight. The telescope thenmust be rotated over the second axis and the vertical anglerecorded.

In order to calculate the horizontal distance D in suchcases, the equations below have been derived from Figure23.

Module 2 – 15


Figure 22

The stadia system

i = Stadia interval

S = Staff intercept

f = Focal length

c = Distance of object glass to instrument axis

o = Outer focal point

D = Horizontal distance from instrument to staff

d = Horizontal distance from outer focal point to staff

Equation 5

D = L x sin�

Where:

L = 100 x S'

S' = S x sin�

Therefore:

L = 100 x S x sin�

It should be noted, however, that this is only true when �is close to 90°. It is then assumed that the upper and lowerlines of sight are parallel to the middle line. Therefore: keep� between 85° and 95°.

Substituting for L in Equation 5 gives:

Equation 6

D = 100 x S x sin2�

D is the reduction of the slope distance L to the requiredhorizontal distance (Figure 23). However, Equation 6 iscumbersome for manual calculations. Therefore, for anglesbetween 85° and 95°, where sin2� is almost 1, Equation 6again reads like Equation 4, or D = 100 x S. Thus, amaximum error of 0.76% is accepted (= 2 x (sin90 -sin85) x 100).

Vertical height

The reduction of the vertical angle is also needed (Figure24).

From Figure 24 it can be derived that:

Tan� =D

or V =D

V tan�

Since: cot��

, therefore: V = d x cot�tan�

Substituting cot� =cos�

results in:sin�

Equation 7

V = D xcos�

sin�

Substituting the formula for D (Equation 6) in Equation 7results in the following:

V = 100 x S x sin2� xcos�

sin�

V = 100 x S x sin� x cos�

Since: sin� x cos� =1

sin2�, it follows:2

Irrigation manual

16 – Module 2

Figure 23

Calculating the horizontal distance

Figure 24

Calculating the vertical height

Equation 8

V = 100

x S x sin2�2

Reduced level

Strictly speaking, the reduced level refers to the elevation ofa point relative to the mean sea level. For example, if theelevation of a point P in relation to a benchmark is 5 m andif the reduced level of the benchmark, which is the elevationof the benchmark above the mean sea level, is 10 m, thenthe reduced level of P, which is the elevation of P relative tothe mean sea level, is 5 + 10 = 15 m. However, in realityit is not always possible to link the elevations of the area tobe surveyed to the mean sea level. This is the case whenthere is no point available, of which the reduced level (theelevation above sea level) is known. In that case, a fictiveelevation can be given to a benchmark, for example 100 m,and the elevation of point P relative to the elevation of thatbenchmark can be measured. If the difference in elevationbetween P and the benchmark is 5 m, then the elevation(reduced level) of P is 100 + 5 = 105 m.

From Equation 8 one can derive the formula for thecalculation of the reduced levels of the points related to thetheodolite position (Figure 25).

The difference in height between the ground level at thetheodolite position (RLi) and the ground level at the staffposition (RLs) is represented by H and:

H + h = V +Hi

or:

Equation 9

H = Hi + V - h

Substituting for V (Equation 8) in Equation 9 gives thefollowing:

Equation 10

H = 100

x S x sin2� + Hi - h2

Sin 2� gives a negative sign when � is more than 90°, whichoccurs when viewing downhill. In such cases the term(100/2 x S x sin2�) becomes negative.

If the reduced levels are included in Equation 10, Equation11 results:

Equation 11

RLs = RLi + Hi ±100

x S x sin2� - h2

Where:

RLs = Reduced ground level at the staff

position (m)

RLi = Reduced ground level at the

instrument position (m)

Hi = Height of the instrument (m)

S = Stadia hair interval or staff intercept (m)

� = Vertical angle in degrees (°),

minutes ('), seconds (")

h = Middle hair reading (m)

Equation 11 holds for theodolites where the stadia constantK equals 100. If this is not the case, the correct constant Kshould be put, which will change the number 100/2.Equation 11 is the formula used in calculating reducedlevels after a tacheometric survey.

Module 2 – 17


Figure 25

Calculating the reduced level

Hi = Height of instrument

h = Middle hair reading

2.3.10. Accuracy

Normal theodolites have enough accuracy for allmeasurements needed for the preparation andimplementation of irrigation schemes.

For altimetry (height measurements), goniometry(horizontal angle measurements) and telemetry (distancemeasurements), several sources of error have to beconsidered. Sources of error include the inaccuracyinherent in the instrument, the graduation of the staff, theverticality of the staff, the inclination, the distance, the mis-centering of the theodolite as well as the staff, weatherconditions and the human eye.

Special attention should be given to the following:

1. The staff reading: if the stadia constant is 100 and thesmallest graduation estimated on the staff is ± 1 mm,there will be ± 10 cm (= 100 x 1 mm) uncertainty inthe horizontal distance. For distances over 100 m, theestimation of the staff intercept will not be any betterthan ± 5 mm, resulting in ± 50 cm of uncertainty inthe horizontal distance. This is the reason why readingdistances should be kept below 100 m, especially forthe stations. If conditions allow, for example favourableweather, intermittent readings could go up to 150 m.

2. Non-verticality of the staff and telescope inclination: acombination of both can amount to a serious error.Therefore, the persons who hold the staff should beinstructed to keep staffs vertical and the telescopeinclination should be kept within ± 5° from thehorizontal line of sight.

3. The vertical angle: this takes into consideration what hasbeen described under (2) and asks for telescopeinclination less than ± 5° from the horizontal line ofsight. If, for example, the actual vertical angle is 85°,which is wrongly read as 84º, the reading error of 1°results in the following horizontal distance error:

For 85°: D = 100 x S x sin285 = 99.24 x S

For 84°: D = 100 x S x sin284 = 98.91 x S

Error: (99.24 - 98.91) x S = 0.33 x S

If the actual angle is 80° but misread as 79°, the errorwould increase to 0.62 x S.

In general, it is accepted that for a distance measuredby the stadia method an error of ± 0.3% will beacceptable (30 cm per 100 m) and that errors onheight differences will be within ± 4 cm.

4. The horizontal angle: miscentering of the theodolite, toolong reading distances, shimmering due to hottemperatures, miscentering the staff on top of the peg,

non verticality of the staff, etc., all contribute to theangular misclosure one can experience when plottingthe traverse of the benchmarks or stations. Horizontalangular misclosure will be discussed in detail in Section2.5.3.

2.4. The level

Level instruments are available in many different modelswith different features, accuracy, etc. Two types commonlyused to take the elevation readings of points in the field arethe tilting level (Figure 26a) and the automatic level (Figure27a). The main difference between the two types is thetilting screw of the tilting level. This tilting screw is used forhorizontal adjustments of the instrument to ensure thatthere is a horizontal line of sight. The automatic level has anautomatic compensator, which is a mechanism thatautomatically gives a horizontal line of sight because of thegravitational force on the compensator.

If the level instruments have horizontal circles one could setout horizontal angles without resorting to the theodolite or,in the case of right angles, to prismatic squares. Thus, suchlevel instruments could be used to set out the grid. Fromthe above it can be concluded that the tilting level is morelaborious to use than the automatic level, and is thereforebecoming less popular.

After mounting either of the instruments on a tripod, onehas to exactly centre the bubble of the circular level with thelevelling screws (Figure 27b and Figure 14). Two levellingscrews are turned at the same time. The one nearest to thecircular level should be turned anti-clockwise and theopposite one should be turned clockwise. When centered,the vertical line of the instrument is actually vertical. Theautomatic level is now ready for use. The tilting level stillneeds horizontal adjustment. Looking through thetelescope eyepiece the coincidence reading level appears onthe left of the field of view (Figure 26b). To centre the levelone has to coincide the tips of the split bubble with thetilting screw. Now the tilting level is also ready for use.

2.5. Tacheometric survey

Tacheometry stands for the Greek word meaning ‘fastmeasure’. ‘Fast’ because heights and distances between theground marks are obtained by optical means only, as sucheliminating the slower process of measuring by tape.

The instrument used for tacheometric surveys is thetheodolite, which has been described in Section 2.3. Thefollowing sections describe the sequential procedure ofcarrying out tacheometric surveys.

Irrigation manual

18 – Module 2

Module 2 – 19


a. Components

1. Spirit level mirror 1. Telescope objective

2. Telescope eyepiece with diopter scale 2. Reflecting surface

3. Tilting screw 3. Horizontal circle

4. Telescope objective 4. Spirit level mirror

5. Focussing knob 5. Telescope eyepiece with diopter scale

6. Horizontal slow motion screw 6. Circular level

7. Tripod head 7. Tilting screw

8. Fastening screw 8. Circle reading magnifier

9. Tripod head

10. Fastening screw

Figure 26

Tilting level (Source: Kern & Co, undated)

1

1

2

2

33

4

4

5

5

6

6

7

78

8

9

10

Not centered Centered

b. Coincidence bubble of a tilting level

Irrigation manual

20 – Module 2

a. Components

1. Base plate 1. Observation prism of circular level

2. Horizontal circle 2. Peep sight

3. Functioning control 3. Focussing knob

4. Eyepiece

5. Adjusting screw cover

6. Circular level

7. Centering point

8. Peep sight

9. Objective

10. Focussing knob

11. Fine motion adjustment screw

12. Levelling screw

Figure 27

Automatic level (Source: Leica, undated)

51

2

3

1

2

3

4

67

9

10

11

12

8

b. Centering circular bubble of an automatic level

2.5.1. The reconnaissance survey

The reconnaissance survey is one of the most importantaspects of any surveying operation and must always beundertaken before any angles or distances are measured.The aim of the reconnaissance survey is to familiarizeoneself with the area and to locate suitable positions forstations. Often people neglect this process, leading towasted time and inaccurate work later. An overall picture ofthe area is obtained by walking all over the site, even morethan once. If plans or maps of the area (or aerialphotographs) exist, these should be consulted. Stationsshould be set out such that each of them can be shot fromas many other stations as possible, while sufficient surveydetail can be obtained from them. The maximum shootingdistance should not exceed 100 m and the distancesbetween stations should be, as much as possible,approximately equal. For most sites, a polygon traverse isusually sited around the perimeter of the area at points ofmaximum visibility (Figure 28).

The stations should be located on firm, level ground so thatthe theodolite and tripod are supported adequately. It is agood habit to draw a sketch of the traverse more or less toscale. The stations should be labelled with reference lettersor numbers. Always indicate clearly the position of astation, as in pasture for example, it can be difficult torelocate them even after only a few hours.

2.5.2. Station marking

After the reconnaissance survey, stations have to be marked.The marks must be semi-permanent or permanent (usuallycast in concrete), not easily moved, and clearly visible.

For intermediate stations, wooden pegs are used, which arehammered into the ground until the top of the peg isalmost even with the ground level. If it is not possible todrive the whole length of the peg into hard ground the,excess above the ground should be sawn off. Too long pegsare often easily knocked off.

More permanent stations require marks set in concrete andlabelling in white paint. It is good practice to make a sketchof all the benchmarks or stations on a traverse in order tocheck if the traverse can close.

2.5.3. The actual survey

The position of a point on the ground can be established ifits angle and distance from another already establishedpoint are measured. During the actual survey this process isextended to successive stations. The resulting series ofconnected lines, of which the angle and distance are known,is called a traverse. Traverse surveying is a method of

control surveying, used to determine the horizontal pointsor rectangular coordinates of control points. There are twotypes of traverses:

� Open traverse: a traverse that does not close on aknown point. Such a traverse cannot be checked andshould therefore be avoided

� Closed traverse: a traverse that starts from onecoordinated point and closes on another coordinatedpoint or on its starting point, forming a polygon(Figure 28)

The station angles and distances should be recorded in thesurvey notebook. It is important that each round ofobservations from a station is completed by closing back tothe initial point sighted (see the example in Table 2). Thefirst and last reading should be compared to verify that theposition of the theodolite has not changed. After thetraverse survey, the data of the traverse stations should beplotted and the correctness checked. Using the 360°graduation, a simple field calculation to check on thecorrectness of the horizontal angles is as follows:

– The sum of the internal angles � of a closed traverse is:�� = (2 x n - 4) x 90°

– The sum of the external angles � of a closed traverse is:�� = (2 x n + 4) x 90°

Where n is the number of stations.

As an example, the angular misclosure of the traverse inFigure 28 is checked. The sum of the surveyed angles in thefigure is 718° and the number of angles is 6. Using theequation for the internal angles:

�� = (2 x 6 - 4) x 90° = 720°, thus the angular

misclosure is 2°

Module 2 – 21


Figure 28

A closed traverse or polygon

The allowable misclosure, E, can be calculated as follows:

Equation 12

E = S x �n

Where:

E = Allowable misclosure

n = Number of traverse stations

S = Smallest reading interval on the theodolite (")

The smallest reading interval on the theodolite is 10", thusthe allowable misclosure is:

E = 10” x �6 = 24"

The allowable misclosure being 24", it can be concludedthat the misclosure of 2° is not acceptable.

When the readings are within the allowable error, thedifference between observed and calculated values isdivided equally between angles, i.e. added to (in case ofnegative angular misclosure) or subtracted from each of theobserved angles.

Only when the traverse survey is in order should theintermittent readings begin to be taken. The theodolite isagain set up over a station and all necessary readings aretaken, including the neighbouring stations and the points inthe selected spacing. Where applicable, the survey shouldalso include the reading of special features (graves, anthills,etc.), existing infrastructure (roads, canals or pipe layouts),water sources, etc.

2.5.4. Data processing

Once all the fieldwork is finished, reduced levels anddistances and coordinates have to be calculated, anddistances and angles plotted.

For those who have access to computers, software isavailable to facilitate and speed up the process of reducing

and plotting. However, even when using computersoftware to calculate reduced levels and to plottopographic maps, it is still good practice to plot by handthe positions of the stations used for the traverse so thatany angular mis-closure can be detected in time.Normally this should been done in the field to avoiddetecting errors back at the office, which would requirereturning to the project area.

Since often people in the field or in remote areas have nocomputer facilities, all computations have to be donemanually, with the help of a pocket calculator. For this, inthe sections below the manual data processing is explainedmore in detail.

Although all the calculations explained below can be donewith any calculator, it can be a laborious exercise.Therefore, programmable calculators can be very useful.Examples of simple programmes to calculate distances andreduced levels are given in Annex 1 at the end of thischapter for Hewlett Packard HP15C, HP42S and TexasInstruments T1-60 calculators.

Calculation of reduced levels and distances

For each point read with the theodolite, the reduced levelis calculated with Equation 11 and the horizontal distancewith Equation 6. If the line of sight is nearly horizontal,Equation 4 can be used instead of Equation 6.

Calculation of coordinates

During the survey, horizontal angles (� or �) anddistances (D) are read. With this information thecoordinates of a Point P (XP, YP) can be calculated. Thecoordinates of a point give the relative position of thatpoint to x and y-axes (Figure 29). In the field angles areread, but for the calculation of the coordinates the bearing(�) is required, which is an angle related to a fixed axissystem (Figure 29).

Irrigation manual

22 – Module 2

Figure 29

Coordinates of point P

The coordinates XP and YP from Figure 29 can becalculated using the following equations:

Equation 13

XP = LOP x sin�

and:

Equation 14

YP = LOP x cos�

Where:

XP = x-coordinate of point P (m)

YP = y-coordinate of point P (m)

LOP = Distance of point P from the origin O

or the length of the straight line OP (m)

� = Bearing or angle of OP relative to the

y-axis

LOP and � are obtained from field measurements using thetheodolite.

As discussed earlier, it is rare that the whole field can besurveyed from one station. Thus a traverse has to be made(Figure 30). The starting point will be station O. The zeroreading is set to the north if there is a compass, otherwiseit can be set to any direction, which from then onwardswill coincide with one of the axes, normally the y-axis,during future plotting. The angle reading to station P, �OPis also the bearing of point P from O. This bearing is usedto determine the coordinates of point P, as illustrated inFigure 30.

The instrument is then moved to station P and read backto station O. This can be done by carrying the bearing(Section 2.3.3) or the return view can be made 0°. Thelatter method makes it a bit easier to calculate the bearing,but there is less possibility to check possible errors. Afterviewing back, the angle between the legs PO and PQ, isread. This could be either the external angle � or theinternal angle � of the polygon, but once one of them hasbeen chosen, the same angle (either external or internal)should be followed throughout the actual survey to avoidconfusion.

From the above, the coordinates for point P can becalculated since the bearing �OP is known. The next step isto find the bearing P to Q or �PQ. From Figure 30 itfollows that:

Equation 15

�PO = �OP + 180°

Equation 16

�PQ = �PO + �P

These equations can be used for the calculation of thebearing for every station. Note that external angles of thepolygon should be used, since that was the direction chosen(see above). Figure 31 shows a case where the internalangles of the polygon are read.

Module 2 – 23


Figure 30

Bearings for part of a traverse

Irrigation manual

24 – Module 2

Figure 31

Traverse with read internal angles, using the 360° graduation (not to scale)

Example 1

Figure 31 shows a traverse for which the internal angles were read. Calculate the bearings at each point.

The external angles can be calculated as follows:

external angle � = 360° - internal angle �

The bearing from A to B is read in the field as �AB = 60°

The bearing from B to A is calculated as follows: �BA = �AB + 180° = 60° + 180° = 240°

The bearing from B to C is calculated using Equation 16 as follows: �BC = �BA + �B

Therefore:

�BC = 240° + (360° - 90°) = 510°

If, at any time in the process of computation, the value of a bearing calculated is greater than 360º, the appropriate

multiple of 360º should be subtracted from it. Similarly, if the values turn out to be negative the appropriate multiple

of 360° should be added.

Thus, the corrected value of �BC will then become:

�BC = 510° - 360° = 150°

Similarly, bearings at all the remaining stations can be calculated using these basic principles:

Bearing of C to D: �CD = �CB + �C = �BC + 180° + �C

�CD = 150° + 180° + (360° - 240°) = 450° - 360° = 90°

Bearing of D to E: �DE = �DC + �D = �CD + 180° + �D

�DE = 90° + 180° + (360° - 80°) = 550° - 360° = 190°

Bearing of E to F: �EF = �ED + �E = �DE + 180° + �E

�EF = 190° + 180° + (360° - 85°) = 645° - 360° = 285°

Bearing of F to A: �FA = �FE + �F = �EF + 180° + �F

�FA = 285° + 185° + (360° - 150°) = 675° - 360° = 315°

There is a need to check the correctness of the calculated bearings by recalculating the bearing of A to B.

Bearing of A to B: �AB = �AF + �A = �FA + 180° + �A

�AB = 315° + 180° + (360° - 75°) = 780° - 360° = 420° - 360° = 60°, which is correct.

After the bearings have been calculated, the coordinates canbe determined using the measured distances betweenpoints and the calculated bearings (see also Figure 30).

Figure 32 shows again part of a traverse.

The bearing �OP and the coordinates of point O areknown. These data are the starting point for the calculationof the coordinates of point P (XP, YP).

Equation 17

XP = XO + �XOP

Equation 18

YP = YO + �YOP

Where:

XP = x-coordinate of point P (m)

YP = y-coordinate of point P (m)

XO = x-coordinate of the starting point

(first station) (m)

YO = y-coordinate of the starting point

(first station) (m)

�XOP = LOP x sin�OP (Equation 13)

�YOP = LOP x cos�OP (Equation 14)

Substituting for XOP in Equation 17 and for YOP inEquation 18 results in:

Equation 19

XP = XO + LOP x sin�OP

Equation 20

YP = YO + LOP x cos�OP

The X and Y coordinates of station P in Figure 32 thusbecome:

XP = 0.0 + 95 x sin60° = 82.3 m

YP = 0.0 + 95 x cos60° = 47.5 m

Before the coordinates of point Q can be determined, thebearing �PQ has to be calculated in the same manner as wasshown in Example 1, which gives:

Bearing of P to Q:

�PQ = �PO + �P = �OP + 180° + �P

= 60° + 180° + (360° - 130°)

= 110°

From Figure 32 it follows that:

�XPQ = LPQ x cos(�PQ - 90°)

= LPQ x sin�PQ, which is Equation 13

�YPQ = -LPQ x sin (�PQ - 90°)

= LPQ x cos�PQ, which is Equation 14

Thus Equation 13 and 14 are valid for any bearing.

Thus:

XQ = XP + �XPQ = XP + LPQ x sin�PQ

= 82.3 + 80 x sin110°

= 157.5 m

YQ = YP + �YPQ = YP + LPQ x cos�PQ

= 47.5 + 80 x cos110°

= 20.1m

Module 2 – 25


Figure 32

Part of traverse and coordinates (not to scale)

The above calculations can easily be made with a simplespreadsheet in, for example, Excel. Table 3 shows the layoutof a spreadsheet programme that can be used to do thecalculations.

The required input data for the spreadsheet are:

� The total number of stations (column 1)

� The station numbers (column 2)

� The internal angles � or external angles � between thetraverse legs (column 3)

� The bearing � of the station 1 (column 4)

� The length of traverse legs (column 5)

� The coordinates of the first station (X1 in column 10and Y1 in column 11)

It is possible to check the errors in the angles of thestations, using the spreadsheet, by simply adding them upto see if the sum conforms with the theoretical sum of(2 x n - 4) x 90° for internal angles or (2 x n + 4) x 90°for external angles, to be put as a formulae in thespreadsheet. If the difference in the sum of the measuredangles of a closed traverse and the theoretical sum, � or�� is acceptable it is distributed equally over the numberof stations.

The spreadsheet automatically calculates the coordinates Xand Y of all stations in the traverse through the formulaeput in the different columns.

It also calculates closing errors X and Y in the X and Ydirections with the formula in the last row of thespreadsheet. The closing error in the coordinates is thedifference between the initial coordinates of the first stationand the calculated coordinates of the same station. These

closing errors are input for the calculation of the linear mis-closure.

Equation 21

Linear misclosure = �X2 + Y2

�L

Where:

X = Closing error in the X direction = ��X (m)

Y = Closing error in the Y direction = ��Y (m)

L = Length of a traverse leg (m)

The value of the linear misclosure is an indicator for theaccuracy of the measurement of the traverse. For sprinklerirrigation systems, the recommended value should be lessthan 0.002 (1/500), while for surface irrigation it should beless than 0.0005 (1/2000). If the linear misclosure isacceptable, then the closing errors X and Y aredistributed proportionally to the lengths of the differentlegs of the traverse, using the following formulae for the xand y coordinates:

Equation 22

ADJ�X = X xL

�L

Equation 23

ADJ�Y = Y xL

�L

Where:

ADJ�X = Factor with which �X needs to be

adjusted

ADJ�Y = Factor with which �Y needs to be

adjusted

X = ��X

Irrigation manual

26 – Module 2

Figure 33

Traverse of four stations and calculation of coordinates

Y = ��Y

L = Length of traverse leg for which DX

or DY have been calculated

�L = Sum of all traverse leg lengths

If the value of X or Y is negative, then �X or �Y shouldbe increased in order to eliminate the error, thus the valuesof ADJ�X or ADJ�Y are positive. If the value of X or Yis positive, then �X or �Y should be decreased in order toeliminate the error, thus the values of ADJ�X or ADJ�Y arenegative.

2.5.5. Plotting

Map scale

A suitable scale for a topographic plan may be 1: 500 to1: 2 000 or smaller, depending on the size of the area undersurvey and the amount of detail required. The scale of1:500 or 1:2 000 can be read as 1 cm on the map is equalto 500 cm in the field or 1 cm on the map is equal to2 000 cm in the field respectively. It is important to use thesame units for the map and the field and only convert to therequired units once a calculation is completed. The scale ofa map is the ratio between distances on the map and actualdistances on the ground. For example, 3.4 cm on a mapwith a scale of 1 : 2 000 means an actual distance in thefield of 3.4 x 2 000 = 6 800 cm = 68.00 m.

Plotting the survey data

Traverses and points can be plotted directly from observedangles and calculated distances. This is done with aprotractor. A protractor is a semicircular instrument withgraduated markings that is used to construct and measureangles. The centre of the protractor is placed on a stationposition such that the observed horizontal angle reading toa known point (usually the previous station) coincides withthe same angle reading on the protractor (Figure 34).Keeping the protractor in place, all other horizontal anglesand distances are plotted, using a ruler for the distances.This plotting includes all stations, intermediate points andspecial feature points. The advantage of this method is thatit is quick and simple. The disadvantage, however, is thatthe method is not very accurate in plotting the angles.

The surveyed points can also be plotted using the calculatedcoordinates. For each point the X and Y coordinates areplotted in a coordinate system. The advantage of thismethod is that adjustments are possible before plotting, asexplained earlier, thus this method is accurate. Thedisadvantage of the coordinate method is that manualcalculation of the coordinates is laborious.

The importance of indicative remarks, noted in the field foreach point, becomes clear here. Remarks like anthill,contour ridge, field edge, corner, tree, road, gullies are ofgreat help to reproduce a close to reality map of the area.

Module 2 – 27


Example 2

Figure 33 shows a traverse of four stations. The bearing � of station 1, the external angles � of all stations and thedistances between subsequent stations L have been surveyed. The coordinates of station 1 are chosen as being(100.00,100.00). What are the coordinates of the other three stations?

Table 4 shows the spreadsheet that has been filled in as follows:

– The number of stations is 4 (column 1)

– The external angles � have been measured in the field and are recorded in the spreadsheet (column 3)

– The bearing of the point of origin �12 has to be fixed in the field and is recorded in the spreadsheet, the other

bearings can be calculated using the formulae put in the spreadsheet (column 4)

– The lengths of the traverse legs L are calculated after the measurements and recorded in the spreadsheet

(column 5)

– �X and �Y are calculated using the formulae put in the spreadsheet (column 6 and 8). The closing errors and

the linear misclosure are calculated using the formulae put in the last row. Since the linear misclosure of 0.0021

is acceptable in this case, the �X and �Y can be adjusted using Equation 22 and 23 (column 7 and 9). For

example, �X12 should be adjusted with:

0.44 x 65

= +0.12 m231

– Finally, the co-ordinates of different stations are calculated using the formulae put in the spreadsheet (column 10

and 11).

Irrigation manual

28 – Module 2

Table 3

Layout of spreadsheet for calculating coordinates using 360° graduation)

No. of Station Angle (a) Bearing � Length L �X ADJ�X �Y ADJ�Y X Y Station

stations number (°) (°) (m) (m) (m) (m) (m) (m) (m) number

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)

Z

1 �1 or �1X1 Y1 1

=known =known�12 = L = �X = (ADJ�X = �Y = ADJ�Y =

read measured (5)xsin(4) (15)x(5) (5)xcos(4) (16)x(5)X2 = Y2 = 2

2 �2 or �2(14) (14)

X1+ Y1

�23 = (6)+(7) (8)+(9)

�12+180+�2=

3 �3 or �3�12+180+(3)(b)

Z

1

Sum of Closing Closing

Sum of lengths error error Linear

angles (14) (15) �16) misclosure

�� or ��Should be

(13)(c) �L X = ��X Y = ��Y �X2 + Y2

Error �L

� or �

(13) - ��or - ��

(a) � = internal angle; � = external angle. � = 360° - �

(b) If the external angle � is measured, then the value in of column (3) can be used as such for the calculation of column (4). If the internal angle � ismeasured and recorded in column (3), then the value of 360° minus the value of column (3) should be used for the calculation of column (4), since � =360° - �.

(c) The sum of internal angles � = (2 x n - 4) x 90° or the sum of external angles � = (2 x n + 4) x 90°, whereby n = number of stations.

Module 2 – 29


Table 4

Calculation of station coordinates using a spreadsheet (using 360° graduation)

No. of Station Angle � Bearing � Length L �X ADJ�X �Y ADJ�Y X Y Station

stations number (°) (°) (m) (m) (m) (m) (m) (m) (m) number

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)

4

1 274 100.00 100.00 1

40 65 +41.78 +0.12 +49.79 -0.06

2 270 141.90 149.73 2

130 62 +47.49 +0.12 -39.85 -0.06

3 293 189.51 109.82 3

243 68 -60.59 +0.13 -30.87 -0.07

4 243 129.05 78.88 4

306 36 -29.12 +0.07 +21.16 -0.04

1 274 100.00 100.00 1

Sum of Sum of Closing Closing

angles lengths error error Linear

�(1-4) misclosure

1 080

Should be

1 080 231 -0.44 +0.23 0.0021

Error �

0

The remarks should be noted on the map. Also should beput:

1. All BMs and TBMs, their elevation and description

2. Water sources and the elevation of the water levels

Tracing the contour lines

Contour lines are traced by interpolating for every 1 m or0.5 m of height difference, depending on the requirements.For example, the contour map for a proposed sprinklerscheme does not have to be as accurate as one for a surface

scheme. Therefore, an interval of 1 m could be chosen.Interpolation by eye is a rather rough method. Thegraphical interpolation is preferred and is explained below:

� A piece of transparent tracing paper is prepared with aseries of equally spaced and scaled horizontal lines, asshown in Figure 35. Every tenth line is drawn heavierthan the others

� The tracing paper is then laid between pairs of spotheights and is rotated until the horizontal linescorrespond to the known spot height values of thepoints (Figure 35)

Irrigation manual

30 – Module 2

Figure 34

Plotting with a protractor

Figure 35

Interpolation between two reduced level points using graph paper

� The heavy lines indicate the positions of the contourlines where they pass over the line joining the spotheights and these positions are pricked through on tothe drawing paper

� The reduced levels (elevations) of all pricked points arewritten down and all points with the same elevation arejoined by smooth curves. For example, all prickedpoints marked 99.0 are joined

As a rule, contours of different elevation do not unite. Theycan exceptionally unite to form one line or cross in the caseof a vertical or overhanging cliff. At steep cliffs the lineswould have to be drawn so close together that they becomeillegible. Furthermore, a single contour line can not splitinto two lines of the same elevation.

Contour maps can also be generated with the automaticcomputer aided design (AutoCAD) programmes, where itis available.

2.5.6. Checking the theodolite

Theodolites should be checked regularly before they aretaken into the field. Some simple checking procedures onthe theodolite are explained here. The nomenclature usedin the text can be related to Figures 12 and 13. If none ofthe adjustments discussed in the next sections correct theproblem, the instrument should be checked by a qualifiedrepair technician.

Adjustment of an inclined vertical reticle line

If the vertical reticle line (hair) is out of plump, inaccuratereadings will result. It is therefore necessary to check thevertical adjustment (Figure 36):

1. Select a clear target and after sighting in the usual way,use the horizontal fine motion screw to position thevertical reticle line exactly on target A.

2. Raise the telescope slightly with the vertical fine motionscrew to position the target between the double verticallines.

3. If the target is off-centre, remove the reticle adjustmentcover. Place a small piece of plastic or wood to one sideof the capstan screw as a buffer, look through theeyepiece and tap the screw gently to position the targetat B. The vertical reticle line is now true.

Horizontal adjustment of the vertical reticle line(360° graduation)

The difference between Face Left and Face Right readingsshould be exactly 180°. Any slight discrepancy is caused bya lateral shift of the reticle in relation to the opticalalignment of the telescope. It is, therefore, necessary tocheck the difference between Face Left and Face Rightreadings (Figures 37 and 38):

1. Select a clear target at a horizontal distance of morethan 10 m.

2. Read the horizontal angle at Face Left, for example a =18°34'00"

3. Take a second reading of the same target at Face Right(turn both horizontal and vertical axes 180°), forexample b = 198° 34' 40".

4. The difference between Face Right and Left anglereadings is 180° 00' 40". The 40" in excess of therequired difference has to be eliminated.

Use the formula: b +a - b ± 180°

2

to obtain the Face Right reading required, reduce theexcess 40" by half. In the formula a = Face Left anglereading and b = Face Right angle reading.

198° 34' 40" + 18° 34' 00" - 198° 34' 40" + 180°

2

= 198° 34' 20"

Module 2 – 31


Figure 36

Adjustment of an inclined vertical reticle line

5. Turn the micrometer knob to position 34' 20" at theminutes and seconds index. The 198° reading at the Hwindow will now be positioned slightly off-centre(Figure 38). Turn the horizontal fine motion screw tore-centre the 198° reading at H. Look through thetelescope. The target will now appear slightly off-centrein the reticle.

6. To eliminate the remaining 20" excess, remove thereticle adjustment cover and turn the left and rightcapstan screws with the adjusting pin to re-centre thetarget in the reticle. The difference between Face Leftand Right readings will now be 180°.

Vertical adjustment of the horizontal reticle line(360° graduation)

This adjustment is required for the correct determinationof the vertical angle (Figure 39):

1. Set up the theodolite in a level position, set the verticalcircle to 90°, erect a staff 20 to 40 m away and read theposition of the reticle against the staff graduations(reading a).

2. Transit (turn around) the telescope vertically, set thevertical circle to 270° and sight the staff to confirm thatthe reticle is centred on the same graduation (reading b).

Irrigation manual

32 – Module 2

Figure 37

Face Left and Face Right readings

Figure 38

Horizontal adjustment of the vertical reticle line

Figure 39

Vertical adjustment of the horizontal reticle line

3. If readings a and b do not coincide, remove the reticleadjustment cover and turn the top and bottom capstanscrews with the adjusting pin to position the reticlehalfway between the reading a and reading bgraduations.

Adjustment of the optical plummet

1. When the theodolite is level and the surveying pointappears in the centre of the reticle, loosen thehorizontal motion clamp screw, turn the theodolitethrough 180º and look at the surveying point again.The surveying point should appear in the centre of thereticle.

2. If the surveying point is out of centre:a) Correct one half of the displacement with the four

optical plummet adjustment screws (Figure 40 andFigure 13). Correct the remaining half by turningthe levelling screws.

b) Rotate the instrument and repeat the adjustmentto ensure that the surveying point is always in thecentre of the reticle.

2.5.6. General remarks

The following practical remarks should be taken intoaccount when carrying out a tacheometric survey:

� Survey more area than required for the design

� Surveying is a job to be carried out with precision. Byits nature, surveying can lead, after some experience,to automatism or robotism. It is at this stage thatmistakes most probably occur. Therefore regularlycheck your numbers, angles and distances in the fieldand make sure to close the traverse

� There is nothing to be gained from hiding errors, asthis does not remove them. They will reappear at a laterstage when dealing with them will be much moredifficult and expensive

2.6. Grid surveys using a level instrument

Grid surveys are often used in areas proposed for thedevelopment of surface irrigation projects. This surveymethod involves the setting out of a grid of points in thearea, for which a contour map is required. After taking staffreadings of all points in the grid and after the calculation ofthe reduced levels (elevations), the elevations are plottedand a contour map is prepared.

During actual implementation of the proposed project thegrid pegs in the field play an important role as they give easyreference points for setting out canal alignments, for theland levelling, etc.

2.6.1. Setting out a grid

The area to be surveyed should be covered by a grid ofpoints at 25 to 100 m intervals for sprinkler irrigationsystems and 10 to 50 m intervals for surface irrigationsystems. The latter irrigation method usually requires acloser grid, as surface irrigation schemes need landlevelling, for which a more detailed contour map is needed.The selected grid interval also depends on the topographyof the area. For large areas with a flat topography the largergrid interval of 50 m could be selected. For areas with steepslopes a closer grid should be chosen in order to reduce thevertical distance between grid points. Once the grid intervalhas been determined, the following method of setting outthe grid should be used (Figure 41).

Setting out a baseline

A baseline is set out on the ground, usually at the edge orthe centre of the area to be surveyed. It is advisable not tosite the baseline in an area that will be disturbed by thedevelopments, as the baseline could then be lost, throughland levelling for example. At distances of the choseninterval, for example 25 m, pegs are placed in a straightline. This can be done accurately with the level instrument,tape and ranging rods. The pegs could be numberedalphabetically. A number of pegs on the baseline should becast in concrete, so that they will be permanently available.It is advisable to take reference points, including some onthe baseline, from the (temporary) benchmark. In this waythe grid becomes permanently fixed and can be set up againbefore actual implementation of the project.

Setting out (grid points) traverses

After setting out the baseline, parallel lines (traverses) at25 m intervals are set at right angles to the baseline using alevel instrument with horizontal scale or a prismatic square.The instrument is placed vertically above each peg on the

Module 2 – 33


Figure 40

Adjusting the optical plummet

baseline, and then turned through an angle of 90°. Pointsare fixed on the ground with pegs at 25 m intervals alongthe traverse. These pegs are numbered numerically (such astraverse D in Figure 41) or given a reference markaccording to their distance from the baseline (such astraverse H in Figure 41).

If the traverse is very long, one has to move the levelinstrument along the traverse. Alternatively, ranging rodscould be used for setting out the traverse once two or threepoints on that line have been established with aninstrument. With ranging rods in a straight line it is easy tosight other rods on points of the traverse still to beestablished (see Section 2.3.1).

If the length of the traverse is shorter than the length of thebaseline, using the following method could save some time(see traverse H in Figure 41).

Place the instrument exactly over a peg on and abouthalfway along the baseline, for example point H0, whichcorresponds with point H of the baseline. After sighting thebaseline, the instrument is turned through 90° and pegs areplaced at 25 m intervals on the H-Line (same procedure as

described above). After this, the instrument is centeredover peg H25 and the grid line A25 to N25 is set out. Thisprocess is repeated for all other lines, A50 to N50, A75 toN75, etc., until the whole area is covered by a grid of pegsat 25 m intervals.

If a more detailed survey, with points for example at 10 mintervals, is required, the interval in the field along thetraverses could be a multiple of 10, for example 30 or50 m, depending on the length of the measuring tape.Then, during the actual survey, the tape is placed betweenthe pegs and intermittent points are read at the correctinterval as indicated with the tape (Figure 42).

Setting out exact angles

For precise angle measurements or setting out exact angles,one should use a theodolite. However, for the setting out ofangles to establish a grid of points in the field one could usea level instrument with a horizontal circle. It is, however,important to set out the angles as accurately as possible. Forexample, if an angle of 88° instead of 90° is set out, theerror becomes significant for longer lines (Figure 43).

Irrigation manual

34 – Module 2

Figure 41

Setting out of grid points

The error is given by the following formula:

Equation 24

Error = L x sin�

Where:

Error = Deviation (m)

L = Length of traverse (m)

� = Error in angle reading (°)

The error in this case is the amount of deviation from theintended traverse line direction.

2.6.2. Checking the level instrument

It is recommended to check the correctness of the levelinstrument before a grid survey starts. The followingprocedure should be followed in order to determinewhether an instrument is faulty (Figure 44).

Module 2 – 35


Figure 42

Reading grid points with the use of a measuring tape

Figure 43

Error experienced when setting out grid lines

Example 3

If L is 50 m and � = 2°, then the error is 50 x sin2 =

1.75 m

If L is 180 m and � = 2°, then the error is 180 x sin2

= 6.28 m

1. Place the instrument exactly between the two points tobe read.

2. The difference in elevation between A and B (h) is AH– BI if the level instrument is okay or AP - BQ if thelevel instrument is faulty. However, both readings willgive the same height difference between A and B.

3. Place the instrument on one side of both staffs (forexample at the right of both staffs, as in Figure 44). Thecorrect reading for the difference in elevation betweenA and B would be h = AK - BG. With a faulty levelinstrument the reading would be h' = AE - BF.

4. Error calculation:LE + AE = h + BF or h = LE + AE - BF

h - h' = LE + AE - BF - (AE - BF) = LE

5. The fault in a level instrument can be adjusted with theadjustment screws. Reference is made to instructionbooklets that are supplied with each level instrumenton how to adjust.

2.6.3. The actual survey

As indicated in Section 2.6.1, a topographic survey shouldalways start from a benchmark, such as position TBM (1)in Figure 41. It is not advisable to make the distancebetween the instrument and the staff too long. Too long adistance makes exact reading of the staff difficult,especially when it is hot or windy. The basic principle ofreading a staff has been explained earlier and is shownagain in Figure 45.

Irrigation manual

36 – Module 2

Figure 44

Checking the level instrument

Figure 45

Principle of staff reading

Taking measurements

Using the level instrument, spot elevations should be takenaround all the pegs in the field. The staff should be placedon a point that typically represents the area around the peg.Thus for example if the wooden peg is on an isolatedanthill, do not take the elevation on top of that anthill buttake a level a bit away from that point. The safest way tocarry out the survey is to start at a benchmark, then surveytwo to four traverses and close either on the samebenchmark as where the survey started or another knownbenchmark in the area. It is advisable to concrete every 4thto 6th peg on the baseline, which could serve as TBMs. Bysurveying a small part of the total area at a time, one canavoid too much repetition in case of a large survey error.Figure 46 shows the principles of surveying, includingopening at a benchmark (AB), change points (CD/CE,JK/JL), intermediate points (FG and HI) and closing atanother benchmark (MN).

Methods used for recording

The next step is to record the staff readings in a surveynotebook and calculate the reduced levels later, which isreferred to as reducing in surveying. Two methods, the riseand fall method and the height of line of collimationmethod, are commonly used for recording and reducingand are explained below.

The rise and fall method

From Figure 45 the centre line reading on the staff at pointA is a and on the staff at point B is b. Therefore thedifference in elevation height between A and B is given by:

hB - hA = BC = a - b

If the centre line reading at point A is larger than that atpoint B (in which case the difference between a and b ispositive), it represents a rise. This means that in that casethe level at point B is higher than the one at point A. On theother hand, if the value of a is smaller than that of b (inwhich case the difference between a and b is negative), itrepresents a fall. This means that the level at point B islower than the one at point A. Therefore, in order tocalculate the level at point B when the level at point A isknown, one has to add a rise to or subtract a fall from thelevel at point A. This can be explained simply with thefollowing expressions:

Level at B = Level at A + Rise or

Level at B = Level at A - Fall

The levels at points A and B are normally referred to asreduced levels (RL) (see Section 2.3.8).

Thus the height level of point B is:

Equation 25

hB = hA + (a - b)

Where:

hB = Level at point B (m)

hA = Level at point A (m)

a = Centre line reading at point A (m)

b = Centre line reading at point B (m)

When the second centre line reading (Forward Station (FS)or Intermediate Station (IS)) is lower than the first centreline reading (Back Station (BS)), it represents a rise. Whenthe second centre line reading is higher than the first centreline reading it is a fall.

Module 2 – 37


Figure 46

The field survey methodology

The height of line of collimation method

In this method, the height of the line of collimation abovethe datum line (for example the TBM) is determined byadding the centre line (staff) reading of the point of aknown elevation to the RL of that point (Barnister andRaymond, 1986). In Example 4, TBM has been establishedto be 100.00 m. The centre line reading at this point,referred to as a, is 2.216 m and therefore the height of theline of collimation becomes 102.216 m (= 100.00 +2.216 m). To calculate the RL at the second point (FS orIS), the staff centre line reading is subtracted from theheight of line of collimation.

Horizontal distance

The difference between upper and lower stadia lines(Figure 45) multiplied by 100 gives the horizontal distancein metres between the staff and the instrument.

It is advisable to make the distances between theinstrument and the staff, such as AD and DB in Figure 45,more or less equal in order to eliminate possible errorswhen there is a deviation in the horizontal line of sight, asexplained in Section 2.6.2 and shown in Figure 47.

Irrigation manual

38 – Module 2

Example 4

Given: a = 2.216 m, b = 1.474 m, hA = 100.00 m (TBM elevation). What is the level at point B?

hB = 100.00 + (2.216 - 1.474) = 100.742 m

In this case a is larger than b and thus the difference between them is positive, which means a rise. This means that

the level at point B is higher than the level a point A.

Given: a = 0.749 m, b = 1.756 m, hA = 100.00 m. What is now the level at point B?

hB = 100.00 + (0.749 - 1.756) = 98.993 m

In this case a is smaller than b, thus giving a fall. This means that the level at point B is lower than the level a point

A.

Example 5

Using the same readings from Example 4, where RLA = 100.00 m, the staff centre line reading at point A is 2.216 mand the staff centre line reading at point B is 1.474 m, what is the level at point B?

Height of line of collimation = 100.00 + 2.216 = 102.216 m

RLB = Height of line of collimation - Centre line reading at point B

RLB = 102.216 - 1.474 = 100.742 m, which is the level at point B

Also using the same readings for the second case from Example 4, where RLA = 100.00 m, the staff centre linereading at point A is 0.749 m and where the staff centre line reading at point B is 1.7456 m, what is the level at pointB?

Height of line of collimation = 100.00 + 0.749 = 100.749 m

RLB = 100.749 - 1.756 = 98.993 m, which is the level at point B

Example 6

Stadia lines readings on point B are:

Upper hair = 1.655 m

Lower hair = 1.294 m

What is the horizontal distance between the instrument and point B?

Horizontal distance: DB = (1.655 - 1.294) x 100 = 36.1 m

The actual recording and data processing

All levels should be written down in a clear order. Table 5shows the field results of part of a topographic survey usingthe rise and fall method. Table 6 shows the results showinga method whereby immediately reduced levels arecalculated (height of collimation method).

After closing on a benchmark the survey error can becalculated quickly by adding up all backward and forwardreadings. The difference should be zero if one closes on the

same benchmark or the difference in elevation between twoknown benchmarks, if one closes on a benchmark differentfrom the one at starting.

The accepted error depends on the accuracy required forthe survey, but in general ranges from 5 to 10 x �L in mm,where L is the surveyed distance in km. In Table 5 and 6the survey distance (L) covered is 0.40 km. The acceptederror in mm is 5 x �0.4 = 3.2 mm. An error of 3 mm isjust acceptable.

Module 2 – 39


Figure 47

Elimination of error due to disparallelism

b. Elimination of disparallelism error by equalling backward and forward distances

a. Error due to disparallelism of line of sight and horizontal line

Example 7

Using the survey data in Table 5, calculate the reduced levels of stations 1, 2, 3 and 4, using the 'rise and fall' method.The height of the TBM is 89.694 m.

The calculations using Table 5 are done as follows:

1) 1.740 - 0.738 = +1.002, which is positive � rise � RL1 = 89.694 (benchmark) + 1.002 = 90.696

2) 0.738 - 2.033 = -1.295, which is negative � fall � RL2 = 90.696 (previous calculated point) - 1.295 =

89.401

3) 1.630 - 1.750 = -0.120, which is negative � fall � RL3 = 89.804 - 0.120 = 89.684

4) 1.553 - 1.694 = -0.141, which is negative � fall � RL4 = 89.684 - 0.141 = 89.543

Irrigation manual

40 – Module 2

Table 5

Extract of Nabusenga irrigation scheme (Zimbabwe) field survey data from survey book: recording using the

rise and fall method

Module 2 – 41


Table 6

Extract of Nabusenga irrigation scheme (Zimbabwe) field survey data from survey book: recording using the

height of line of collimation method

Irrigation manual

42 – Module 2

2.6.4. Plotting

The results of the survey have to be plotted on a map, afterwhich the contour lines can be drawn, as explained inSection 2.5.5 (Figure 48).

2.6.5. Use of pegs during implementation

Once the contour map is ready, design work for theirrigation project is carried out. If the project is going to beimplemented soon after the survey and design phases, onemight still find the grid of pegs intact in the field, which willplay an important role in setting out canal alignments, pipealignments, etc. This can often be done with limited use oftheodolites or level instruments with horizontal circle as thepegs give easy reference points.

Example 8

Using the survey data in Table 6, calculate the reduced levels of stations 1, 2, 3, 4, 5 and 6, using the 'height of lineof collimation' method. The height of the TBM is 89.694 m.

The calculations in Table 6 are done as follows:

1) RL1 = 89.694 (benchmark) + 1.740 (back sight) = 91.434

2) RL2 = 91.434 - 0.738 = 90.696

3) RL3 = 91.434 - 2.033 = 89.401

4) RL4 = 91.434 - 1.630 = 89.804

5) RL5 = 91.434 - 1.750 = 89.684

89.684 + 1.553 = 91.237

6) RL6 = 91.237 - 1.694 = 89.543

Figure 48

Contour map

Annex 1

Programmes for the calculation of reduced levels intacheometric surveys for programmable calculatorscommonly available

Programmable pocket calculator HP 15C

The starting point for the programme is Equation 11 for

the calculation of reduced levels, explained in Section

2.3.9:

RLs = RLii + Hi + 0.5 [(UH - LH) x100] x sin 2A - MH

RL, H, UH, LH and MH are expressed in metres and A in

degrees, minutes and seconds. The reduced level of the

station where the instrument is set up (RLi) and the height

of instrument (Hi) are constant for each reading station.

They can be stored and should be changed each time the

instrument is moved to another station.

Suppose we store RLi in cell 5 � STO 5 and Hi in cell 6

� STO 6

The upper hair (UH), middle hair (MH), lower hair (LH)

and vertical angle (A) are variables for each shot taken.

They have to be entered while the programme is running.

The order in which they have to be entered is:

– UH

– MH

– LH

– A

While running the programme, the first figure to appear is

the slope distance divided by 100. The second figure is

the reduced level.

The programme to be entered in the calculator is:

– f LBL A

– R/S (input of upper hair UH)

– STO1

– R/S (input of middle hair (MH)

– STO2

– R/S (input of lower hair (LH)

– STO3

– R/S (input of vertical angle A)

– STO4

– � H (to read angle in decimals)

– 2

– *

– SIN

– 0.5

– *

– RCL1

– RCL3

– -

– R/S (read slope distance * 100)

– *

– 100

– *

– RCL5

– +

– RCL6

– +

– RCL2

– -

– GTO A (read reduced level)

– GP/R

Programmable pocket calculator HP42S

Before the actual programme is entered, the variables

should be stored in the memory. To run the programme,

use the <XEQ> key and input the variables one by one.

After every entry press <R/S) key to move to the next

variable. The following is the basic programme and can

be improved by making loops or single inputs for

constants.

The entering of the programme should be done as

follows:

– 0

– STO"RLs"(create storage capacity for variables)

– STO"RLi"

– STO"Hi"

– STO"UH"

– STO"MH"

– STO"LH"

– STO"AN"

– •GTO.. (following three key strokes necessary to

initiate programming)

– •PRGM

– •PGM.FCN

– LBL"TACH" (gives programme an unique name)

– INPUT"RL"

– INPUT"HI"

– INPUT"UH"

– INPUT"MH"

– INPUT"LH"

– INPUT"AN"

– RCL"AN"

– 2

– *

– SIN

– 50

Module 2 – 43


Irrigation manual

44 – Module 2

– *

– RCL"UH"

– RCL"LH"

– -

– *

– RCL"RLi"

– +

– TCL"Hi"

– +

– RCL"MH"

– -

– STO"RLs"

– VIEW"RLs"

– END

Programmable pocket calculator TI-60

1. Make enough memory available by pressing <2nd

part 6>

2. Clear the programme space by pressing <2nd CP>

3. To start programming press <LRN>

4. In this case HI and RL1 are added and stored in 0,

i.e. Hi + RL1 ? STO 0.

5. The order in which variables have to be entered is:

– Vertical angle

– UH

– LH

– MH

The programme to be entered reads as follows:

– R/S (input vertical angle)

– 2nd DMS-DD (to change angle into decimals)

– *

– 2

– =

– SIN

– STO 2

– (

– R/S (input UH)

– *

– 100

– -

– R/S (input UH)

– *

– 100

– -

– R/S (input LH)

– *

– 100

– =

– STO 1

– )

– RCL 2

– *

– RCL 1

– ÷

– 2

– +

– RCL 0

– -

– R/S (input MH)

– =

– R/S (read reduced level)

– RCL 1 (read slope distance)

– RST

To get out of the programme mode, press <LRN>again.

Module 2 – 45

In soil science, soil texture refers to particle size or therelative amounts of sand, silt and clay. These primary soilconstituents play an important role in drainage, nutrientfertility, compactability, elasticity, freeze-thaw behaviour,groundwater recharge, adsorption of pollutants, and manyother properties that are relevant to agriculture,development, planning, suitability analyses, environmentalstudies, and biogeography (Byron, 1994). Soil consistencyrefers to the strength and nature of the cohesive forceswithin a soil and the resistance of the soil to mechanicaldisintegration, deformation and rupture. Consistencydepends largely on the soil texture, especially the claycontent. It also depends on the moisture content of the soil.Soil structure refers to the aggregation of primary soilparticles (sand, silt, clay) into compound particles orclusters of primary particles, which are separated from theadjoining aggregates by cracks or surfaces of weakness. It ischaracterized by the shape and size of the aggregates as wellas by the distribution of pores brought about by thisaggregation (Euroconsult, 1989). More detailedinformation on soil texture and structure is given inModule 4.

The aim of any soil survey is to produce a soil map, showingmap units that are delineated on the basis of soilcharacteristics, as well as a soil survey report, dealing withland and cultural characteristics of the area underconsideration.

Soil surveys are, as a rule, undertaken by soil scientists. Forsmall projects, however, irrigation specialists at times mayundertake the soil surveys. This chapter covers the verybasic elements of soil surveys for irrigation purposes. For anin-depth study on soil surveys the reader is referred to FAO(1979). As individual countries may have their own systemsfor land classification, the reader is also advised to consultwith the relevant Land Use Planning Services.

To judge a soil, one has to observe its profile, which isnormally characterized by a succession of layers orhorizons, describe the pits and take samples for laboratoryanalysis.

3.1. Field observations

The objective of the field survey is the identification ofsignificant areas of each type of soil in an area. Descriptions

of soil characteristics serve as the basis for soilidentification, classification and interpretation.

3.1.1. Soil pit description

Depending on the scale of the soil map to be drawn, anumber of pit profiles have to be described, backed by anumber of auger holes. At least three auger holes per hashould be drilled and one soil pit for each different soil typeif the scale does not exceed 1 : 5 000, which is normally thecase for irrigation projects (Ministère de la coopérationfrançaise, 1980).

Before a soil pit description can start, representative pitshave to be identified. A photo-interpretation map ortopographical map can be taken into the field wheretraversable survey routes identified in the office can bechecked on the ground. Doing so can reduce the amount offieldwork substantially, since areas not being worth furtherinvestigation can be left unsurveyed.

Once the variation in land and soil conditions and theirdistribution are roughly understood, survey routes shouldbe selected as perpendicular as possible to expectedboundaries between different soil types. Surveying intransitional zones and parallel to their direction should beavoided, as these zones are not representative of a given soilunit. Soil pits should be selected at the most characteristicsites of the soil units and their location should be indicatedon the topographic map for later references.

Field equipment required for a soil survey includes:

� Munsell (colour) handbook

� Water flask

� Auger

� Chisel

� Towel

� Soil packs

� Measuring tape

Soil pits are described using the following parameters:depth, texture, colour, structure, permeability and limitingmaterial, such as gravel and crust material. These effectiveparameters are used to distinguish the soil horizons. The

Chapter 3Soils and soil surveys

Irrigation manual

46 – Module 2

effective depth of the soil profile is determined by thebiological activities visible along the depth of the soil profile,by the limiting materials and by the depth of the watertable.

All this information is compiled into a form giving thestandard code description. Using the relevant landcapability and irrigability classification system, a class can bederived. Some classifications distinguish five classes, namelyA, B, C, S and D, with class A being the most suitable forirrigation and class D being unsuitable for irrigation(Thompson and Purves, 1979).

It should be noted, however, that this classification holdsmore for surface irrigation systems than for pressurizedirrigation systems. It is based on a 5 to 6 day irrigationinterval, the slope of the land, the water-holding capacityand the soil permeability. Mild slopes and soils with higherwater-holding capacity are ranked higher. As a rule, higherdepths of water application and less frequent irrigationcombined with gentle slopes are suited to surface irrigation.However, the built-in management of pressurized systemscombined with their flexibility for very frequent irrigation(even daily in the case of drip systems) allows irrigation oflight soils (with lower water-holding capacity) and higherslopes (up to 4%). Therefore, lower irrigability classes ofsoil or marginal soils can be easily irrigated with thesesystems.

An example of a soil pit description is given in Table 7.

3.1.2. Augering

Augering is mostly used to complement information on soilpits. It is a quick way of checking the extent of the soil unitaround the profile. A record of the depths of the variouslayers and their estimated content of organic matter, soiltexture, colour, their estimated content of organic matter,soil texture, colour, consistency, concretions etc., aresufficient for establishing changes in soil profile propertiesthrough augering.

3.1.3. Soil sampling

In order to complete and confirm the visual fieldobservations, soil samples are collected. Some principles ofsampling are:

� In the soil pit, each horizon is made accessible so thatsamples can be taken easily. Of each horizon, at leasttwo but preferably three samples are taken (totallingabout 1 kg of material from each freshly exposedhorizon), depending on the efficiency of the laboratory

� Each sample should be put in a sample bag and samplebox, clearly indicating the place name, date, soil pitnumber, horizon, depth and name of the observer.These details should appear on the box and on/in thebag

� For mechanical analysis disturbed samples areadequate. For determining the water-holding capacityundisturbed soil samples are needed and specialsampling cylinders have to be used

Table 7

Example of a soil pit description

Date 2 May 1991

Pit number 6

Position 30 m South of the gate along the access road

Vegetation, topography, Tall, fairly thick Msasa woodland. Fairly sparse grass cover. Gently undulating. Virgin

cropping history

Effective depth 70-80 cm to a gravel band on weathering rock

Profile characteristics (texture, Topsoil: Clay of crumb structure. Heavy body. 5YR3/4. Upper subsoil: Sub-angular blocky

permeability, structure, colour, clay. 2.5YR3/4. Permeability slightly restricted. Lower subsoil: Angular blocky clay. Slight

etc.) mottling. 2.5YR3/4. Permeability slightly restricted. Limiting material: Loose gravel band

20 cm thick on greenstone schist

Other features ("t" factors, etc.) Slight tendency of compaction

Land characteristics 1-1.5% slope

Erosion None visible

Wetness A little mottling above the gravel suggests slight wetness.

Code3F43Z/2F

A-1/Gs

Class II

General remarks A representative pit of this area. A good maize soil. Derived from greenstone of the

Basement Complex. Mostly formed in situ.

Module 2 – 47


� Sampling, as well as pit description, is preferably doneafter the rains have finished and when the soil hasdried out.

3.1.4. Infiltration test

Infiltration tests are used to determine the infiltration rateand the cumulative infiltration of a soil, described inModule 7. Of particular interest is the basic infiltration rate,which indicates the flow velocity in a saturated soil. Typicalbasic infiltration rates are given in Table 8.

Table 8

Typical basic infiltration rates

Soil type Basic infiltration rate

(mm/hour)

Clay 1-7

Clay loam 7-15

Silt loam 15-25

Sand loam 25-40

Sand > 40

The infiltration rate should always be greater that the flowrate of the sprinkler used in case of a pressurized system, inorder to avoid ponding and possible runoff of irrigationwater (Module 8). In surface irrigation, it is an importantdata directly related to the intake opportunity time. More

information on infiltration rates, and especially infiltrationtests, is given in Module 7.

3.2. Laboratory analysis

For the purpose of designing an irrigation system, the twomost important soil analyses to be carried out are themechanical analysis and the determination of the water-holding capacity (WHC).

3.2.1. Mechanical analysis

Mechanical analysis serves to determine the particle sizedistribution in the soil, or its texture, by sieving andsedimentation. The sedimentation method is based on thelaw of Stokes: different sized sediment particles insuspension have different sedimentation times. The largerfractions will settle first, the smaller particles last.

From the results of the mechanical analysis, one can findin the texture diagram, or the USDA soil texturaltriangle, the texture class of the soil (Figure 49).Knowing the percentage of clay, silt and sand, lines aredrawn as shown in Figure 50 (dotted lines). For example,a soil containing 30% sand, 30% clay and 40% sand isclassified as a clay loam. Figure 51 shows two methodsfor generalizing soil texture classes (a less detailed and amore detailed one).

Figure 49

USDA soil texture triangle (Adapted from: Fitzpatrick, 1980)

Irrigation manual

48 – Module 2

3.2.2. Water-holding capacity or total available soilmoisture

Based on the data on soil texture, a first estimate of thewater-holding capacity of the soil can be found in theliterature. The water-holding capacity of a soil or theavailable moisture is defined as the difference between fieldcapacity (FC) and permanent wilting point (PWP).

Field capacity is defined as the condition in a soil wherefree drainage of fully saturated soil took place for about 1

to 2 days and the maximum amount of water that aparticular soil can temporarily hold. Depending on soiltype the soil moisture at FC is held with a tension of 0.1-0.3 atmosphere (bars). The lighter the soil the lower thesoil tension.

The permanent wilting point of a soil is the conditionwhere the suction force of plant roots can not overcome thetension of 15 atmospheres (bars) and the remaining wateris held around the soil particles. Sand can store less water

Figure 50

Soil texture class determination, using the USDA soil texture triangle

Figure 51

Two methods for generalizing soil texture classes (Adapted from: FitzPatrick, 1980)

Module 2 – 49


than clay or loam but, put under a slight pressure, sandreleases the water more easily than clay or loam. It shouldbe mentioned that the structure also plays a role: well-aggregated soil can store water in between the macro-poresof the aggregates.

The determination of the available moisture requires thedetermination of the FC and the PWP. They are bothdetermined in the laboratory using the standard pressureplate technique. Cores of soil are wetted to saturation.Pressure would then be exerted until no more drainagewater is measurable. In the case of FC, the pressure wouldbe 0.1 atmosphere for light soils, 0.15 for medium soils and0.3 for heavy soils. In the case of PWP, the pressure will be15 atmospheres. At the end of the test, the wet soil coresare weighed and oven dried at 105°C for 24 hours and thenreweighed. The moisture content is then expressed aspercent of the dry weight of the soil:

Equation 26

SMw =

Wet mass (weight) - Oven dry mass

(weight) x 100

Oven dry mass (weight)

Where:

SMw = Weight moisture content

For irrigation purposes it is always preferable to express themoisture content on a volumetric basis. Bulk volumeconsists of the volume of the soil particles (solid phase) andthe volume of the pores or pore space. The weight of thebulk volume consists of the weight of the soil particles (solidphase) and the weight of the soil moisture. Porosity isdefined as the ratio of pore space to total bulk volume. Toconvert the moisture content from weight basis tovolumetric basis, the bulk density of the soil is required,which refers to the weight of a unit volume of dry soil,which includes the volume of solids and pore space(kg/m3). Thus, the bulk density is determined by weighingthe soil contained in a certain volume. This is the reason forsampling cores of soil. The following expression providesthe bulk density:

Equation 27

Ds = Mass (weight) of dry soil

Bulk volume of soil

Where:

Ds = Soil bulk density

To convert the percentage of moisture from weight tovolume basis the following equation is used:

Equation 28

SMv = SMw x Ds

Dw

Where:

SMv = Soil moisture by volume

SMw = Soil moisture by weight

Ds = Soil bulk density

Dw = Water density

Since Dw = 1, the equation is simplified to:

SMv = SMw x Ds

Uniform plant root development and water movement insoil occur when soil profile bulk density is uniform, acondition that seldom exists in the field. Generally, soilcompaction occurs in all soils where tillage implements andwheel traffic are used. Compaction decreases pore space,thus decreasing root development, oxygen content, watermovement and availability. Other factors that affect bulkdensity include plant root growth and decay, wormholesand organic matter. Sandy soils generally have bulk densitiesgreater than clayey soils.

Having determined the moisture content at FC and PWP,the water-holding capacity of the soil or the total availablesoil moisture on a volumetric basis can be provided throughthe following expression:

Equation 29

SMta-v = SMv (0.1-0.3 bar) - SMv (15 bar)

Where:

SMta-v = Water-holding capacity by

volume (%)

SMv (0.1-0.3 bar) = Soil moisture by volume at

FC (pF � 2) (%)

SMv (15 bar) = Soil moisture by volume at

PWP (pF = 4.2) (%)

The SMta expressed in % can be expressed in mm/m bymultiplying the SMv percent by 10. Table 9 gives a range ofavailable soil moistures for different soils, while Figure 52gives typical pF curves for sand and clay. Often SoilMoisture Tension (SMT) is indicated in pF, where pF is thenegative logarithm (cm water column) and 1 000 cm watercolumn is 1 atmosphere. The right y-axis of Figure 52shows the Equivalent Pore Diameter (EPD). A specific poresize distribution of a given soil determines the specificrelationship between its pF values and the correspondingmoisture contents by volume, since at each pF level allpores wider than the corresponding critical EPD are empty.

Irrigation manual

50 – Module 2

Often, irrigation engineers find it convenient to use tablesrather than waiting for the laboratory test on the values ofFC and PWP. Such an approach should, however, beavoided. The range of available moisture within eachtextural class, as shown in Table 9, is too large to provide anaccurate design basis. The tables should be used onlyexceptionally and such tables should have been derivedfrom previous within-country tests. This can be moregreatly appreciated by comparing the figures in Table 9with those in Table 10.

Table 10

Available moisture for different soil types (Source:

Withers and Vipond, 1974)

Soil type Available moisture (mm/m)

Sand 55

Fine sand 80

Sand loam 120

Clay loam 150

Clay 135

The differences are especially big with the heavy and lightsoils.

3.3. Soil map and soil report

From the topographic map, the field observations and thelaboratory results, a soils map can be drawn, indicating thedifferent soil types with their area, the location of the soilpits, rock outcrops, gravel patches, etc. The soil reportcomprises a general description of the area with averageslopes, indications of erosion, present vegetation, parentmaterial, etc.

Moreover, a classification is given according to thecountry’s soil classification as well as a standard codedescription.

Example 9

A soil has a soil bulk density of Ds of 1.2. After drying 120 grams of wet soil in an oven at 105-110°C for 24 hours,this soil lost 20 grams of moisture.

– What would be the moisture volumetric content of the soil?

– What would be the corresponding water depth in mm/m?

SMw = (120 - (120-20))/100) x 100 = 20%

SMv = 20 x 1.2 = 24%

Water depth = (24/100) x 1000 = 24 x 10 = 240 mm/m

Table 9

Range of average moisture contents for different soil types (Source: Euroconsult, 1989)

Textural class Field capacity Permanent wilting Water-holding capacity (WHC) WHC or

(FC) point (PWP) or available moisture available moisture

(Vol %) (Vol %) (Vol % = mm/dm) (mm/m)

Sandy 10-20 (15) 4-10 (7) 6-10 (8) 60-100 (80)

Sandy loam 15-27 (21) 6-12 (9) 9-15 (12) 90-150 (120)

Loam 25-36 (31) 11-17 (14) 14-19 (17) 140-190 (70)

Clay loam 31-41 (36) 15-20 (17) 16-21 (19) 160-210 (190)

Silty clay 35-46 (40) 17-23 (19) 18-23 (21) 180-230 (210)

Clay 39-49 (44) 19-24 (21) 20-25 (23) 200-250 (230)

Module 2 – 51


Figure 52

Typical pF curves for silty clay and loamy fine sand (Source: Euroconsult, 1989)

Irrigation manual

52 – Module 2

Module 2 – 53

Cropping requires an assurance that there is enough wateravailable at the water source throughout the growing seasonand that the conveyance system can provide the water to thefields, adequate in volume and command.

If water availability is low, an appropriate cropping patternand planting time has to be considered. Where possible,crops requiring minimum water should be grown duringthe dry season and times of peak crop water requirementsfor different crops should be spaced and should notcoincide with the period of low water availability at thesource.

The water used for irrigation can be either surface water orgroundwater. Irrigation water can be abstracted from rivers,lakes, dams/reservoirs, springs, shallow wells or deepboreholes. Another source of water is the so-called non-conventional source of water, which includes treatedwastewater and desalinated seawater. Although on a verylimited scale, some countries in East and Southern Africause treated wastewater for landscape irrigation or irrigationof non-edible crops, or they return the effluent to the watersupply reservoir after tertiary treatment. Desalinatedseawater is not used at all yet in the sub-region in view ofthe high cost of desalinization. It is used in some countriesof the Middle East and the Mediterranean basin, thoughmainly for municipal/domestic purposes.

Most countries in East and Southern Africa still have a biastowards the assessment and development of surface waterresources as compared to groundwater resources and thishas resulted in a considerable amount of surface waterinformation being collected.

4.1. Water yield levels

A country like Zimbabwe is reasonably well endowed withwater. However, only a small portion of the rainfall, usuallyless than 10%, appears as flow in the river systems, the restbeing ‘lost’ to evaporation, transpiration or replenishmentof groundwater.

There are considerable variations in water availability, bothwithin a year and over the years. To be of any value, aconstant water supply must be sustained, with a stated riskof failure. In Zimbabwe, a risk of 4% is generally employedfor primary (municipal/domestic) purposes. This means

that the failure to supply the quoted yield of water will be 4in 100 years or 1 in 25 years. For irrigation purposes, a riskof 10% is used, meaning that the failure to supply thequoted yield of water will be 1 in 10 years. This is a ratherconservative figure. Worldwide, a risk of 20% foragricultural use is generally acceptable, implying a failure tosupply the quoted yield 1 in 5 years. Lower risk factorsimply lower yields, since lower yields can be supplied withless risk than higher yields. Lower yields result in lowerlevels of investments in irrigation infrastructure (damconstruction, conveyance systems, etc.), since the area thatcan be irrigated is less. Higher risks translate into higheryields and this could act as an incentive in irrigationinfrastructure investment, thereby transforming the socio-economic status of most people, in particular thebeneficiary rural communities. However, the risk factorshould be carefully weighed against the benefits.

Conservative (low) risk factors lead to a lower totalutilization of the water as less baseflow can be used (rivers)or a greater proportion of water held in storage to carryover with consequent higher evaporation losses from dams.It should be noted, however, that the yield at 10% risk givesgreater security against short-term shortages than the yieldat 20% risk.

Yield versus dam capacity curves can be constructed forvarious risk factors. These are asymptotic and there is anoptimum yield obtainable for a certain dam capacity andany increase in dam capacity would not result in anysignificant increase in the yield. It is thus not cost effectiveto over-design a dam.

4.2. Rivers

Rivers or streams with a regular and certain minimum flow(baseflow) are suitable for irrigation. Unfortunately, manyrivers in Southern Africa have short duration flash floodsduring the rainy season and no or very little flow during thedry season (Figure 53). These rivers are not suited for yearround irrigation, unless the water can be stored in areservoir behind a dam.

The hydrograph of river A shows that the base flow at 10%risk is 1 m3/sec, thus this flow could be diverted throughoutthe year. River B is seasonal and irrigation can only takeplace during the rainy season between November and

Chapter 4Surface water resources

Irrigation manual

54 – Module 2

March at a safe abstraction of about 200 1/sec. However,reservation for other purposes (municipal, industrial,environmental) also has to be considered.

The feasibility of using rivers for irrigation can be determinedby a statistical analysis of long-term river flows. For mostmajor rivers, these data are available from the departments ororganizations responsible for hydrological data such as theMinistry of Forestry and Water Affairs in South Africa orNational Water Authorities in other countries.

For most smaller rivers no flow measurements are available.It is thus difficult to determine the water flow during thegrowing seasons. Nevertheless, a clear indication is needed,especially during the latter part of the dry season whenminimum river flow normally coincides with maximumevapotranspiration. There are ways of obtaining some ideaabout the flow regime, such as by talking to local (preferablyelder) people, visiting the area during the dry season,analyzing satellite imagery data (remote sensing) and bycarrying out flow measurements with current meters orisotope and salt dilution methods. Whether data areavailable or not, one has to come up with a safe water yield,which in turn determines the possible irrigation area. Oncethis is known, one should apply for an appropriate waterright or water abstraction permit from the relevantauthority in the country.

It is equally important to have knowledge of high floods inorder to properly design diversion structures and floodprotection works near the river. Again, it is useful to talk tothe local people, who can often indicate flood marks, forexample on trees.

Many rivers carry large amounts of sediments especiallyduring the rainy season. This has to be verified and, if so,the designs of the headworks have to cater for sedimentflushing arrangements to avoid it entering the canal system.

The stability of especially meandering rivers has to beconsidered in order to avoid placing headworks in unstableparts of the river.

4.3. Dams and reservoirs

Where rivers do not provide sufficient baseflow forirrigation, storage structures could be built in order tobalance river flows, not only throughout the year but alsoover sequences of several years.

4.3.1. Sedimentation

The amount of sedimentation depends on many aspects,including soil type, climate, slopes, vegetation cover,deforestation, livestock, population pressure andmanagement practices in the catchment area of a dam.Sedimentation can cause serious problems to dams,particularly small ones, or weirs, as the reservoirs could fillup rapidly. A simple calculation of how to determinesedimentation of dams is shown later.

The source of all sediment is the land in the catchment areaof the dam. Sediment that enters the river system istransported either as bed load or as suspended load. Bed loadcomprises the larger (sand) particles that are swept along orclose to the riverbed. This type of load accounts forapproximately 10% of the total sediment in the river.

Figure 53

Streamflow hydrographs

Module 2 – 55


Suspended load includes all finer particles like silt and clay.These materials are carried in suspension and will only settledown when the flow is slowed down, for example in areservoir created by a dam (Figure 54). In general, the bedload is deposited first at the tail end of the reservoir, afterwhich respectively the heavier and lighter suspendedmaterials settle.

Sometimes fine mud settles out on top of the coarsermaterials at the end of the flood season since the flow, inmost cases, will be very much reduced. The mud isrelatively impermeable, which can cause impermeablelayers with no free movement of water between the layers,thus resulting in the river completely drying up during thedry seasons. There are cases where small reservoirs behinddams and weirs are filled with sand and alluvium, whichwould still allow abstraction of water, as approximately 30%of the reservoir volume remains filled with water. In suchcases, abstraction can be done through sand abstraction. Aseries of screens or slotted pipes are buried below the watertable in the sand and attached to a pump, which pumps thewater from the sand. It should be noted, however, thatdams are not constructed to be used for sand abstraction.Sand abstraction schemes are mostly carried out inriverbeds with significant amounts of sand or alluvium.

The reservoir trap efficiency is a measure of the proportionof the total volume of sediment that is deposited in areservoir to that which enters the reservoir. The totalvolume of sediment entering a reservoir each year will bethe product of the sediment concentration in the water, themean annual runoff and the catchment area:

� Sediment concentration (SC): This depends on how wellpreserved the catchment area is. Three categories areoften used, namely sediment concentrations of3 000 mg/l (3 kg/m3), 5 000 mg/l (5 kg/m3) and10 000 mg/l (10 kg/m3).

� Catchment area (CA): This is the total land areacontributing runoff into the reservoir (km2).

� Mean annual runoff (MAR): This is the average net runoff,expressed as a depth of water over the dam’scatchment area (mm).

The mean annual inflow into the reservoir (MAI) isexpressed as follows:

Equation 30

MAI = CA x MAR

Where:

MAI = Mean annual inflow into the reservoir

(m3)

CA = Catchment area behind the dam

(m2)

MAR = Mean annual runoff (m)

The trap efficiency is related to the gross storage ratio,which is expressed as follows:

Equation 31

SRg = DC

MAI

Where:

SRg = Gross storage ratio

DC = Gross dam capacity (m3)


(m3)

For large dams with a gross storage ratio of at least 0.10, thetrap-efficiency is 100%, as it is assumed that all thesediment will be settled (Figure 55). For very small dams,there will be almost continuous spilling and only the bedload will settle, thus the trap efficiency will be 10%.

Figure 54

Sedimentation in a reservoir created by a dam

Irrigation manual

56 – Module 2

The above assumes that no measures have been taken toavoid sediments from entering the reservoir, for examplethrough the construction of a silt trap, which would be agood solution if it is desilted or cleaned regularly, or a smalldam upstream of the main dam, which would serve as a silttrap.

It can be calculated that for dams with a gross storageratio smaller than 0.10, approximately 50% of thecapacity is lost due to sedimentation in 20 average yearswhen the river sediment concentration is 5 000 mg/l. Fora sediment concentration of 10 000 mg/l, 50% of thecapacity will be lost within 10 years. Therefore, it shouldbe avoided to construct reservoirs with a storage ratiosmaller than 0.10.

The mass of sediments entering the reservoir each yearthrough the river water is expressed as follows:

Equation 32

SM = MAI x SC

Where:

SM = Sediment mass entering the reservoir

annually (kg)


(m3)

SC = River sediment concentration (kg/m3)

For dams with a storage ratio exceeding 0.10 and with ariver with a sediment concentration of 5 000 mg/l, it can becalculated that the sedimentation in 20 years approximates6.5% of the mean annual inflow. This volume should be

deducted from the reservoir gross capacity in order to beable to irrigate a given area for at least 20 years, withoutbeing forced to reduce the irrigation area due to reducedyield.

The volume of sediments depositing in the reservoir everyyear can be calculated using the following equation:

Equation 33

SV = SM

Where:

SV = Sediment volume deposited in the

reservoir annually (m3)

SM = Sediment mass entering the reservoir

annually (kg)

= Density of deposited sediments

(kg/m3)

4.3.2. Dam yields

The dam yield (Q) is defined as the volume of water in m3

that can be drawn from a reservoir behind a dam for useeach year, at the designated risk level.

The following parameters are used in the estimation of damyield:

� Dam catchment area CA (km2)

� Mean annual runoff MAR (mm)

� Gross mean annual inflow into the reservoir MAI: theproduct of CA and MAR (m3)

Figure 55

Trap efficiency

Module 2 – 57


� Evaporation E: the annual net water loss from a freewater surface (mm)

� Maximum reservoir surface area A: the surface area ofreservoir when water is at full supply level (ha)

� Coefficient of variation CV: a mathematical measure ofthe variability of runoff from year to year. It is the ratioof standard deviation of annual inflow to the meanannual inflow. A low CV indicates regular inflow andhigh chances of meeting a particular yield and,conversely, a high CV implies that the chances ofmeeting a particular yield are less. CV can be expressedin % or in decimals

� Net storage ratio SRn: the ratio of live storage capacityU to gross mean annual inflow MAI

Equation 34

USRn =

MAI

Where:

SRn = Live storage ratio

U = Live storage capacity (m3)

MAI = Mean annual inflow into the reservoir (m3)

The live storage capacity is defined as:

Equation 35

U = DC - DS - SA

Where:

U = Live storage capacity (m3)

DC = Gross dam capacity (m3)

DS = Dead water storage below the outlet level

(water which can not be abstracted) (m3)

SA = Sediment allowance over a chosen period

(m3)

The catchment area and the maximum reservoir surfacearea can usually be determined from maps with contourlines at a scale of 1:50 000 for example. The storagecapacity of the dam could also be determined from suchmaps, although a reservoir survey often has to be carriedout to obtain more accurate data on the storage capacity.

Inflow characteristics consist of the MAR and CV of theannual runoff. In most countries, estimates of MAR and CVare given for each sub-catchment area or hydrological sub-zone. An example of such data for Zimbabwe is given inTable 11.

Example 10

Given:

– Catchment area (CA) = 148 km2

– Mean annual runoff (MAR) = 40 mm– Gross dam capacity (DC) = 1 700 000 m3

– Sediment concentration (SC) = 5 000 mg/l or 5 kg/m3

– Density of deposited sediments (d) = 1 550 kg/m3

What is the volume of the reservoir that is lost yearly to sedimentation?

– Gross mean annual reservoir inflow (MAI) = (148 x 106) x (40 x 10-3) = 5 920 000 m3

–Gross storage ratio (SRg) =

1 700 000 = 0.29

5 920 000

� Trap efficiency = 100%, since the storage ratio > 0.1

The deposit of sediment in an average year in kg will be equal to the gross mean annual inflow in m3 multiplied by

the sediment concentration.

Thus, the mass of sediments in the inflowing river water per year is:

SM = (5.92 x 106 m3) x (5 kg/m3) = 29.6 x 106 kg

The volume occupied by the sediment per year is:

SV = 29.6 x 106

= 19 100 m3

1 550

This is the volume of reservoir or water lost to sedimentation yearly.

Irrigation manual

58 – Module 2

Tab

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Module 2 – 59


In our example of Zimbabwe, if the net annual evaporationis not available from direct measurements, it can beestimated at 1 800 mm per annum or 1 350 mm over anine month dry season. In the calculations of the damyields, only nine months of evaporation are used as it isassumed that over a period of the three rainy seasonmonths the evaporation is compensated by inflow.

When all these data are available, the yield at 10% risk canbe calculated as shown in Example 11, utilizing Table 11. Itis noted that the method shown in Example 11 is onlysuitable for dams with a net storage ratio SRn below 0.5.The method for higher storage ratios will be discussed later.

The evaporation index is defined as follows:

Equation 36

El = E x A x 104

U

Where:

EI = Evaporation Index

E = Evaporation over the dry months (m)

A = Reservoir surface area (ha)

U = Live storage capacity in (m3)

The method described in this example does not apply tosituations where SRn is above 0.5. In those cases, themethod discussed in Example 11 can be used to estimatethe reservoir yields. This method is described in moredetail in MEWRD (1984) It makes use of five sets of yieldcurves (Figure 56). These curves, for different CVs, havebeen computed for constant annual draw-off, assuming thatall inflow into the dam takes place during the first threemonths of the hydrological year (the rainy season) and thatthere is no usable inflow for the remainder of the year.

Table 12

Yield/Live Storage Ratios (MEWRD, 1988)

Evaporation Index Net Storage RatioYield/Live Storage (Q/U) Ratio at 10% Risk

(EI) (SRn = U/MAI) CV = 0.8 CV = 1.0 CV = 1.2

0.1 0.88 0.88 0.68

0.2 0.63 0.63 0.52

0.2 0.3 0.54 0.54 0.46

0.4 0.49 0.49 0.43

0.5 0.46 0.46 0.42

0.1 0.83 0.83 0.63

0.2 0.78 0.58 0.46

0.3 0.3 0.62 0.48 0.41

0.4 0.53 0.43 0.38

0.5 0.48 0.41 0.37

0.1 0.72 0.72 0.52

0.2 0.67 0.47 0.36

0.5 0.3 0.51 0.38 0.31

0.4 0.43 0.34 0.29

0.5 0.39 0.31 0.28

0.1 0.61 0.61 0.43

0.2 0.56 0.38 0.27

0.7 0.3 0.42 0.29 0.22

0.4 0.34 0.25 0.20

0.5 0.30 0.22 0.20

0.1 0.47 0.47 0.29

0.2 0.42 0.25 0.15

1.0 0.3 0.28 0.17 0.11

0.4 0.22 0.13 0.09

0.5 0.17 0.10 0.09

Irrigation manual

60 – Module 2

The data required for the computation of the dam yield Qare:

� Mean annual inflow into the reservoir MAI = CA xMAR (103 m3)

� Coefficient of variation of annual inflow CV (%)

� Evaporation factor EF, defined as:

Equation 37

(e x A)3

EF = (0.7 x U)2

Where:

EF = Evaporation factor

e = Net evaporation per year, which is the

annual evaporation minus minimum rainfall

(m)

A = Reservoir surface area at full supply level

(ha)

U = Full supply or live storage capacity (103 m3)

After the net storage ratio (SRn) and the MAI/EF ratio havebeen calculated, Figure 56 is used to determine the Q/MAIratio, after which the yield at 10% risk can be calculated.

Example 11

A dam along a river in sub-zone Z2 in Lower Gwayi sub-catchment (Table 11) within the Gwayi Catchment inZimbabwe has the following characteristics:

– Gross dam capacity (DC) = 1 700 000 m3

– Reservoir surface area (A) = 51.6 ha

– Catchment area (CA) = 148 km2

– Mean annual runoff (MAR) = 40 mm (sub-zone Z2, Table 11)

– Coefficient of variation (CV) = 1.2

– Sedimentation concentration (SC) = 5 000 mg/l or 5 kg/m3

– Sediment allowance 20 years (SA) = 6.5% of MAI

What is the dam yield at 10% risk?

– Gross mean annual inflow (MAI) = (148 x 106) x (40 x 10-3) = 5 920 000 m3

– Gross storage ratio (SRg) =1 700 000

= 0.295 920 000

– Trap efficiency = 100%, since storage ratio > 0.10

– Sediment allowance (SA) (20 yrs) = 0.065 x 5 920 000 = 384 800 m3

– Live storage capacity (U) = 1 700 000 - 384 800 = 1 315 200 m3

– Net storage ratio (SRn) =1 315 200

= 0.225 920 000

– Evaporation Index (EI) =1.35 x 51.6 x 104

= 0.531 315 200

Substituting the SRn and EI values into Table 11 shows that the yield/live storage (Q/U) ratio at 10% risk is

approximately 0.34 for CV = 1.2. This figure is obtained by double interpolation. The Q/U ratio is first calculated for

EI = 0.5 and 0.7 for SRn = 0.22 (by interpolation of the Q/U ratio). Secondly, the Q/U ratio is also interpolated for

EI = 0.53:

EI = 0.5 and SRn = 0.22 � Q/U ratio = 0.36 - (0.22 - 0.20)

x (0.36 - 0.31) = 0.35(0.3 - 0.2)

EI = 0.7 and SRn = 0.22 � Q/U ratio = 0.27 - (0.22 - 0.20)

x (0.27 - 0.22) = 0.26(0.3 - 0.2)

EI = 0.53 and SRn = 0.22 � Q/U ratio = 0.35 -(0.53 - 0.50)

x (0.35 - 0.26) = 0.34(0.7 - 0.5)

Thus, the yield at 10% risk for a yield/live storage ratio of 0.34 is calculated as follows:

Q = 0.34 � Q = 0.34 x 1 315 200 = 447 168 m3

U

Module 2 – 61


Figure 56

Yield curves for dams with storage rations greater than 0.5

Notes

– Risk level = 10%

– 3 months constant inflow followed

by 9 months of no flow

– U = Full supply capacity

– MAI = Mean Annual Inflow

– Q = Volume of draw off/year

(103 m3)

– EF = Evaporation Factor = 1.0 for

average dam with a net evaporation

rate of 1 500 mm/year

Example 12

Given:– Live storage capacity (U) = 274 000 x 103 m3

– Reservoir surface area (A) = 2 030 ha– Annual evaporation = 2 000 mm– Annual minimum rainfall = 400 mm– Mean annual inflow (MAI) = CA x MAR = 250 000 x 103 m3

– Coefficient of variation (CV) = 80%.

What is the dam yield at 10% risk level?

– e = 2 - 0.4 = 1.6 m

(1.6 x 2 030)3

– EF =(0.7 x 274 000)2

= 0.93

– SRn =274 000 x 103

= 1.10250 000 x 103

–MAI

=250 000 x 103

= 269 000 x 103 m3

EF 0.93

QReading the calculated values in the bottom right curve of Figure 56 (CV = 80%) gives a ratio for

MAI = 0.53

Therefore the yield at 10% risk is:

Q = 0.53 x (250 000 x 103) m3 = 132 500 x 103 m3

Irrigation manual

62 – Module 2

Example 13

What is the dam yield of Example 12 at 4% and 20% risk levels respectively?

Q at 4% risk = (132 500 x 103 x 0.9) - (0.03 x 250 000 x 103) = 55 750 x 103 m3

Q at 20% risk = (132 500 x 103 x 1.03) + (0.06 x 250 000 x 103) = 151 475 x 103 m3

Once the yield at 10% risk has been calculated with any ofthe above methods, the yield at 4% and 20% risk can beestimated by the following rules of thumb:

Equation 38

Q at 4% risk = (Q at 10% x 0.9) - (0.03 x MAI)

Q at 20% risk = (Q at 10% x 1.03) + (0.06 x MAI)

Where:

Q = Dam yield (m3)

MAI = Mean annual inflow into reservoir (m3)

Most dams are used for both primary (municipal) andirrigation purposes. As these purposes have different risklevels (see Section 4.1), calculations have to be made to

determine how much water will be available for irrigationat 10% risk, taking into account the municipal waterrequirements at 4% risk.

The equation to use is:

Equation 39

WAirr =Q at 10% risk

x (Q at 4% risk - water need

Q at 4% riskfor primary purposes at4% risk)

Where:

WAirr = Water availability for irrigation per year

(m3)

Q = Dam yield (m3)

Example 14

Given:

– Q at 10% risk = 447 168 m3

– MAI = 5 920 000 m3

– Primary water demand = 200 m3/day or 73 000 m3 per year

What is the water available for irrigation at 10% risk?

– Q at 4% risk is: (447 168 x 0.9) - (0.03 x 5 920 000) = 224 851 m3

– WAirr at 10% risk is:447 168

x (224 851 - 73 000) = 301 990 m3

224 851

Module 2 – 63

Groundwater is an important source of water supply fordomestic, industrial and agricultural purposes if it occurs inadequate quantities of appropriate quality. It is invariablycrucial for semi-arid to arid regions as, in most cases, it formsthe only source of potable water. In Sub-Saharan Africa, mostrural communities rely on groundwater for their safe dailywater needs. While there is a general notion thatgroundwater is the primary or main source of water in semi-arid to arid regions, with surface water being the main sourcein humid regions, this is not that true. In Europe, forexample, groundwater plays an important role as a source ofmunicipal water supplies and even for irrigation purposes.

The utilization of groundwater resources preceded theknowledge and understanding of its occurrence anddynamics. Lack of such information inevitably resulted ingroundwater overexploitation (mining), in some instancesaccompanied by the deterioration of its quality. Theappreciation of groundwater as an important resourceemanates from the understanding of the hydrologic cycle.

5.1. Groundwater resources and thehydrologic cycle

Groundwater is an important and integral part of thehydrologic cycle. Thus it cannot be developed withoutpaying heed to other components of the cycle, as this couldresult in the upsetting of the water balance, possibly leadingto disastrous environmental and human effects.

The interdependence and continuous circulation of allforms of water between ocean, atmosphere and land isknown as the hydrologic cycle (Figure 57).

It is apparent from Figure 57 that a catchment must beenvisaged as a combination of both surface drainage area andthe subsurface soils, and the underlying geologicalformations.

Figure 58 provides the diagrammatic introduction to thehydrologic terminology. The rectangular boxes representstorage and the hexagonal boxes represent water movement.

Table 13 shows data that reflect the quantitative importanceof groundwater relative to other components of thehydrologic cycle.

The oceans and seas comprise 94% of the earth’s totalwater volume. This water is highly saline and can only beput to use after employing expensive desalinizationprocesses. Removing this water from consideration wouldleave groundwater accounting for about two thirds of thefresh water resources of the world. Considering theavailability of the fresh water (minus the icecaps and theglaciers), groundwater would comprise almost the totalvolume. However, average residence times tend tocompromise the volumetric superiority, as they are quitehigh in certain instances such as deep groundwater. Thespatial distribution of groundwater will be looked at undergroundwater occurrence (Section 5.2).

Chapter 5Groundwater resources

Figure 57

The hydrologic cycle

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5.2. Groundwater occurrence

The occurrence of groundwater is controlled, inter alia, bythe prevailing geological conditions. Groundwatercomprises all water that infiltrates and saturates subsurfacegeological formations. Aquifers form important sources ofgroundwater.

5.2.1. Aquifers

An aquifer is defined as a saturated geological formationthat is permeable enough to yield economic quantities ofwater. Aquifers are often called by their stratigraphic(geological sequence) names.

Unconsolidated sedimentary aquifers

Unconsolidated sedimentary aquifers comprise loosesedimentary units such as unconsolidated sand and gravel,products of riverine deposits known as alluvial or fluvialdeposits. They occur in old river channels or flood plains ofmajor rivers and tend to be localized. Examples ofunconsolidated deposits can be found in the Save valley,along the Save river in the south eastern part of Zimbabwewhere the aquifer is used for irrigation in the Middle Sabiand Musikavanhu communal areas. In South Africa, thedeposits can be found, among other river channels, alongthe Brak River. Alluvial fans, aeolian and glacial deposits alsocomprise unconsolidated sedimentary aquifers. Figure 59shows typical alluvial deposits in a flood plain.

Figure 58

Systems representation of the hydrologic cycle (Adapted from: Freeze and Cherry, 1979)

Table 13

Estimate of the water balance of the world (Source: Nace, 1971)

Parameter Surface area Volume Volume Equivalent Residence time

(km2) x 106 (km3) x 106 (%) depth (m)*

Oceans and seas 361 1 370 94 2 500 ��4 000 years

Lakes and reservoirs 1.55 0.13 <0.01 0.25 ��10 years

Swamps <0.1 <0.01 <0.01 0.007 1-10 years

River channels <0.1 <0.01 <0.01 0.003 ��2 weeks

Soil moisture 130 0.07 <0.01 0.13 ��2 weeks-1year

Groundwater 130 60 4 120 ��2 weeks-10 000 years

Icecaps and glaciers 17.8 30 2 60 10-1 000 years

Atmospheric water 504 0.01 <0.01 0.025 ��10 days

Biospheric water <0.1 <0.01 <0.01 0.001 ��1 week

* Computed as though storage were uniformly distributed over the entire surface of the earth

Module 2 – 65


The porosities of unconsolidated sands and gravels rangefrom 30-50% and their permeabilities (hydraulicconductivities) range from 10-4-10-6 m/s.

Consolidated sedimentary aquifers

Consolidated sedimentary formations consist of indurated

or hardened (either through pressure and/or temperature)sediments such as sandstone and carbonate rock.

In many countries sandstone formation forms extensive orregional aquifers with significant quantities of potablegroundwater. Figure 60 shows an idealized sandstoneaquifer.

Figure 59

Development of deposits (unconsolidated sedimentary aquifer) in a flood plain (Adapted from: Driscoll,

Figure 60

An idealized sandstone aquifer (consolidated sedimentary formation) (Source: Driscoll, 1986)

Figure 61

Schematic illustration of groundwater occurrence in carbonate rock with secondary permeability and

enlarged fractures and bedding plane openings (Source: Freeze and Cherry, 1979)

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Carbonate rocks comprise limestone and dolomiteconsisting mostly of the minerals calcite and dolomite withvery little clays. Figure 61 shows a schematic illustration ofgroundwater occurrence in carbonate rocks. In fracturedcarbonate rocks, successful and unsuccessful wells can existin close proximity (Figure 61). It is thus imperative thatgeophysical investigations be carried out to locate thefracture zones. Examples of carbonate aquifers are theLomagundi Dolomite Aquifer in northwestern Zimbabwe(Mhangura-Chinhoyi area) and the GemsbokfonteinDolomite in Far West Rand, South Africa.

Crystalline rocks comprise mostly of igneous andmetamorphic rocks and are usually solid. Weathering andfracturing increase the porosity and permeability of therocks. One of the most characteristic features of thepermeability of crystalline rocks is the general trend ofpermeability decreasing with depth. This will result in thewell yields also decreasing with depth. Consequently, deepwells in such geological formations are not warranted.

5.2.2. Aquifer types

Aquifers are generally bounded either on top or bottom orboth by either an aquitard or aquiclude. An aquitard is ageological unit that is more or less permeable. The unit maybe permeable enough to transmit water in significant

quantities when viewed over large areas and long periods,but its permeability is not sufficient to justify drillingproduction wells. Sandy clays and loams are typicalexamples. An aquiclude is an impermeable geological unitthat does not transmit any water at all. Silt clays, denseunfractured igneous or metamorphic rocks are typicalaquicludes. In nature, strictly impermeable geological unitsseldom occur. They all transmit water to some extent andshould therefore be classified as aquitards.

There are basically three main types of aquifers: confined,unconfined and leaky (Figure 62).

Confined aquifer

A confined aquifer is bounded above and below by anaquiclude (Figure 62A). The groundwater pressure isusually higher than the atmospheric pressure and if a well isdrilled into the aquifer, the water level will rise to above thetop of the aquifer. In certain cases, where the water rises toabove the ground or surface level, the well is referred to asa free flowing or artesian well. The level at which thegroundwater level rises to is known as the piezometricsurface. If the piezometric surface falls below the base ofthe confining aquiclude during pumping, it means that theaquifer can not sustain the discharge.

Figure 62

Various types of aquifers

Module 2 – 67


Unconfined aquifer

An unconfined aquifer, also known as a water table aquifer,is bounded below by an aquiclude and has no overlyingconfining layer (Figure 62B). Its upper boundary is thewater level, which rises and falls freely. Water in a wellpenetrating an unconfined aquifer is at atmosphericpressure and does not rise above the water table. Often insedimentary formations, an unconfined aquifer exists abovea confined one, giving a multi-layered aquifer (Figure 62E).

Leaky aquifer

A leaky aquifer, also known as a semi-confined aquifer, is anaquifer whose upper boundary is an aquitard and the lowerone an aquiclude (Figure 62C and 62D). It can also be amulti-layered leaky aquifer system, mentioned above(Figure 62E). Because the vertical hydraulic conductivity ofan aquitard is more important than its transmissivity,groundwater is free to flow through the aquitard, eitherupwards or downwards, depending on the elevations of therespective water levels in the overlying and underlyingaquifers. Leakage occurs from an aquifer with a water levelat higher elevation to the one whose water level is at a lowerlevel, considering the same datum line or reference point.Leaky aquifers are common is sedimentary formations.

5.3. Groundwater resourcesdelevelopment

The development of groundwater resources, coming afterthe assessment (matching available water resources with thedemand for the water) and the planning (integration ofsupply and demand), is a sequential process with variousphases. Firstly, there is an exploration stage in which surfaceand subsurface geological and geophysical techniques arebrought to bear on the search for suitable aquifers.Secondly, there is an evaluation stage that encompasses themeasurement of hydrogeological parameters, the designand analysis of wells and the calculation of well yields.Thirdly, there is an exploitation or development stage and lastly,a management stage (see Section 5.6).

5.3.1. Groundwater exploration

Although groundwater can not be seen on the surface, avariety of techniques can provide information concerningits occurrence and even its quality. Geophysical explorationmethods are used either before or during wellconstruction and, regardless of the method used, theefficacy of each relies on the contrasting physical andphysical-chemical properties of the groundwater and theaquifer material.

Surface geological methods

The initial steps of a groundwater exploration programmeare carried out in the office rather than in the field (deskstudy) and is a rather inexpensive process. This will involvea desk study focusing on the collection, analysis andhydrogeological interpretation of available geological,topographical and hydrogeological maps, existing reports,aerial photographs, lithological logs and other pertinentrecords. Remote sensing data are also very crucial inhydrogeological studies. Desk studies should becomplemented by field reconnaissance. Knowledge of thedepositional and erosional events in an area may indicatethe extent and regularity of aquifers. The type of rock willsuggest the magnitude of expected water yields.

Subsurface geological methods

It is seldom sufficient to look only at the superficialmanifestations of a hydrogeological environment. It isnecessary to carry out test drilling, for irrigation watersupplies projects, so as to better delineate subsurfaceconditions. Test holes provide the opportunity forgeological and geophysical logging and to obtaingroundwater samples for chemical analysis. They alsoprovide the elevation of the water level. The analysis of thechemical results would give the water quality and itssuitability for irrigation purposes.

Surface geophysical methods

Surface geophysical methods provide specific informationon the stratigraphy and structure of the local geologicenvironment as well as aquifer properties. Stratigraphicdata may include the types and extent of alluvial depositsand the nature and extent of the underlying bedrock(aquitard or aquiclude). Sets of data collected by surfacegeophysical methods are called surveys. Structural featuressuch as faults, fractures, folds, karstic terrain and intrusionssuch as dykes can be located and identified by geophysicalmethods.

There are various geophysical methods that can beemployed in groundwater exploration, such as the electricalresistivity method, the seismic refraction method and thegravity method. The first one being the most commonlyused, will be discussed below.

Electrical resistivity method

This method is widely used in Southern Africa because it ischeap in comparison to other methods such as seismicrefraction.

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The principle of the resistivity method is based on thedifferent abilities of various rock formations to conductdirect electrical current. The passage of the electricalcurrent through the ground depends on the nature of thesoil, rock type, moisture content or groundwater and itssalinity. Geophysical investigations are carried out using theSchlumberger array, whose configuration is given in Figure63. Direct current, I, is introduced into the ground throughouter electrodes A and B. The electrical signal experiencesa potential drop, �V, which is measured between a secondpair of inner electrodes, M and N, as a result of the groundbeing resistive to the passage of the electrical current.

The resistance of the ground R is obtained from Ohm’s law,which states that the ratio of the potential drop isproportional to the electrical current and hence R = �V/I.This resistance can be converted to a resistivity value using ageometric factor K, which depends on the electrodeconfiguration. By continually expanding outer electrodessymmetrically away from the midpoint, O, of the array, thecurrent penetrates deeper and deeper and so does the depthof investigation. Owing to the heterogeneity of the ground,the resistivity measured is in fact an apparent resistivity. A log-log plot of the apparent resistivity against the currentelectrode spacing AB/2 produces the so-called VerticalElectrical Sounding (VES) curve. The sounding curve isinterpreted in terms of layer thicknesses with correspondingresistivity values (often referred to as layer parameters) thatare used to determine the aquifer geometry.

Interpretation of the measurements is done by comparingthe resulting curve (semi-log plot of apparent resistivityversus current electrode spacing) against publishedtheoretical curves for simple layered geometrics. Softwareexists on the market that can assist in electrical resistivity datainterpretation, for example Reinvert and Resixp. The softwarecan only produce good results if one has good data and reallyunderstands the prevailing geological conditions and thelimitations of the methodology. Various lithological layers can

be mapped using the method. It can also be used to locatethe salt water-fresh water interface in coastal aquifers.

Lateral profiling provides areal coverage at a given depth ofpenetration since the electrode separations are kept fixed ateach point of investigation along a profile line. The methodis used to define aquifer limits or map areal variations ingroundwater salinity.

Subsurface geophysical methods

Geophysical well logging, also known as well geophysics or down thehole logging, involves lowering sensing devices into a well andrecording a physical parameter that may be interpreted interms of aquifer characteristics; groundwater quality, quantityand movement; or physical structure of a borehole. Thegeophysical logs include resistivity, spontaneous potential,natural gamma and gamma-gamma logging.

5.3.2. Water wells

In order for groundwater to be put to use, wells have to bedrilled into aquifers to bring it to the surface. A well is ahole or shaft, usually vertical, excavated or drilled into anaquifer and is properly designed and constructed. Wellsmay be hand dug, driven or jetted in the form of wellpoints, bored by an auger or drilled by a drilling rig. Theselection of the drilling method hinges on such questions asthe purpose of the well, the hydrogeological environment,the amount of water required, the borehole depth anddiameter envisaged, and the economic factors.

A number of wells, not widely spaced, drilled to tap thesame aquifer so as to produce significant amounts of water,for example for irrigation, are referred to as a well field.

Test holes

Before drilling a well in a new area, it is common practiceto initially drill a test hole of a much smaller diameter. The

Figure 63

Schlumberger Vertical Electric Sounding (VES) electrode configuration

Module 2 – 69


purpose of a test hole is to determine depths to thegroundwater, groundwater quality and the physicalcharacter and thickness of aquifers without the expense ofa regular well, which might be unsuccessful. Lithological(geologic) logs should be collected and down the holelogging, if possible, carried out.

Drilling in semi-consolidated and consolidated (hardrock) formations

Drilling in such formations requires air-drilling systems.Compressed air (from a compressor) is piped down thedrilling pipes and lifts the rock cuttings and cools thedrilling bit. If the rock is competent, which means if it doesnot collapse or cave, the hole will be left open or uncased.Air drilling is carried out in formations such as crystallineand carbonate aquifers. In carbonate aquifers, such aslimestone and dolomite, possibilities of drilling into karsts(open holes) exist whereupon loss of air circulation will beexperienced.

Drilling in unconsolidated formations

Drilling in unconsolidated formations requires thestabilization of the hole as the drilling progresses. This caneither be done by driving casing as the drilling progresses orby using a type of drilling mud that will seal off the hole andstabilize it for short periods. Drilling that employs mud iseither direct mud rotary or reverse mud rotary drilling. Themud must be biodegradable so as to enable water to enterthe well after construction. Drillfloc and Magnfloc areexamples of such a mud. Bentonite mud is notrecommended as a drilling mud, since it is notbiodegradable and is difficult to clean away after the drillingof the well. In Zimbabwe, the direct mud rotary drillingtechnique is used in drilling irrigation wells in alluvialdeposits.

Well casing

The casing is of a smaller diameter than the drilled hole.The casing keeps the hole open and provides structuralsupport against caving as well as sealing out surface waterand any undesirable groundwater such as salinegroundwater. The casing is usually placed against the non-water bearing formations and is usually made of alloyed orstainless steel. The latter is quite expensive. PVC can also beused but is not recommended for deep holes as it is likelyto break. In shallow wells (generally no more than 50 m) itis recommended to install thicker PVC.

Screens

In consolidated formations, where the materialsurrounding the well is stable, groundwater can enterdirectly into an uncased well. In unconsolidatedformations, however, wells have to be installed withscreens. The screens stabilize the sides of the hole, preventsand movement into the well and allow a maximumamount of water to enter the well with minimum hydraulicresistance. Screens are usually placed against the waterbearing formations.

Manufactured screens, such as the Johnson, are preferredto perforated casing because of the ability to tailor openingsizes to aquifer conditions and the larger percentage ofopen area that can be achieved. Several types are available:punched, stamped, louvered, wire wound perforated pipeand continuous slot wire wound screens. The latter type ismost efficient. It possesses the largest open area and can beclosely matched to aquifer material (Figure 64).

Hacksaw-cut or torch-cut screens are not recommendedfor irrigation wells as they usually have uneven and largeopenings, which will result in the well silting.

Figure 64

Continuous slot wire wound screen in an unconsolidated formation (Source: Todd, 1980)

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The selection of the screen diameter should be made on thebasis of the desired well yield and aquifer thickness. Tominimize well losses and clogging, groundwater entrancevelocities should be within specified limits (Table 14).

Table 14

Optimum groundwater entrance velocity through a

well screen (Source: Walton, 1962)

Hydraulic Optimum Hydraulic Optimum

conductivity screen conductivity screen

of entrance of entrance

aquifer velocity aquifer velocity

(m/d) (m/min) (m/d) (m/min)

>250 3.7 80 1.8

250 3.4 60 1.5

200 3.0 40 1.2

160 2.7 20 0.9

120 2.4 <20 0.6

Hydraulic conductivity, K, is defined as the volume of waterthat will be transmitted through a porous medium in unittime under a unit hydraulic gradient through a unit areameasured perpendicular to the flow direction. Hydraulicconductivity is sometimes referred to as permeability andhas units of velocity (m/day). It depends on a variety ofphysical factors, including porosity, particle size anddistribution, shape of particles, arrangement of particles,etc.

The formula used in the calculation of the optimum screenentrance velocity is:

Equation 40

QVs =

c x � x ds x Ls x P

Where:

Vs = Optimum screen entrance velocity

Q = Well discharge (pumping rate)

c = Clogging coefficient, usually taken to be 0.5

since 50% of the screen open area is

estimated to be blocked

ds = Screen diameter

Ls = Screen length

P = % of open screen area (available from

manufactures)

Gravel pack

Gravel pack is artificially emplaced in the annulus betweenthe screens and hole walls to:

i) Stabilize the aquifer

ii) Minimize sand pumping

iii) Permit the use of a large screen slot with maximumopen area

iv) Provide an annular zone of high permeability, whichincreases the effective radius and yield of the well

Maximum grain size of a pack should not exceed 1.0 cm,while the pack thickness should be between 8 and 15 cm.A natural pack can also be developed around the screenfrom the sand or gravel forming the aquifer material byremoving the fine material through well development (seebelow).

Sanitary seal

A sanitary protection of cement grout is placed near thesurface, and in the annular space between the casing andwall sides, to prevent entrance of surface water and toprotect against exterior corrosion. If the groundwater isalso used for human consumption, this will help inprotecting its quality.

Well development

A new well is developed to increase its specific capacity(well’s productivity), prevent sanding and obtain maximumeconomic well life. Development procedures includepumping, surging, use of compressed air and addition ofchemicals in limestone or dolomite formations. Acombination of these methods is often used.

5.3.3. Collector wells

Collector wells are large wells, up to about 5 m in diameter,usually constructed in unconsolidated formation such asalluvial deposits. If a collector well is constructed adjacentto a surface water source, well yields will be generally highand discharge values of up to 30 000 m3/day are notuncommon. The well is sunk, usually by use of drillingmachinery, to the requisite depth in the aquifer byexcavating inside a cylindrical concrete caisson. Perforatedpipes, 15-20 cm in diameter, are jacked hydraulically intothe aquifer through precast portholes in the caisson to forma radial pattern of horizontal pipes, 60-100 m in length(Figure 65). During construction, fine grained material iswashed into the caisson so that natural gravel pack (seeabove) formed around the perforations. The number,length and radial pattern of the collector pipes can be variedto obtain maximum yields; usually more pipes are extendedtowards than away from the surface water source.

The caisson acts as a large storage tank and one or twopumps can be installed. The large area of exposedperforations in a collector well causes low inflow velocities,which minimize incrustation, clogging, and sand transport.

Module 2 – 71


The initial cost of a collector exceeds that of a vertical well;however, advantages of large yields, reduced pumping headsand low maintenance costs are factors to be considered.

Collector wells can also be developed in aquifers removedfrom surface water sources. In Zimbabwe, collector wellshave been developed in crystalline rock aquifers to enhanceyields for domestic water supplies because ordinary smalldiameter wells had failed to cope with the demand,particularly during drought periods.

5.3.4. Groundwater resources evaluation

Groundwater resources evaluation focuses on, inter alia, theassessment of hydrogeologic parameters, the design andanalysis of wells and the calculation of aquifer yields.Pumping or exploitation of groundwater leads to water leveldeclines, which serve to limit yields. One of the primary goalsof groundwater resources evaluation must therefore be theprediction of hydraulic head drawdowns (Section 5.4) inaquifers under proposed pumping schemes. Suchinformation can be obtained from understanding wellhydraulics (Section 5.4), which looks at the calculation ofaquifer parameters discussed in Section 5.2.

Figure 65

Collector well located near a surface water body

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5.4. Pumping tests

Pumping tests are field measurements used to gather datafor the calculation of transmissivity, storativity, specific yield,leakance, well efficiency (Section 5.4.7) and the predictionof yields and drawdowns.

5.4.1. The principle

When a well is pumped, groundwater is removed from thearea surrounding the well, and the water table (inunconfined aquifers) or the piezometric surface (inconfined aquifers), is lowered. The drawdown, at a givenpoint, is the distance to which the water level is lowered(Section 5.4.2) (Figure 66).

A drawdown curve shows variation of the drawdown withdistance from the pumping well. In three dimensions, the

drawdown curve describes a conic shape, known as the coneof depression. The outer limit of the cone of depression, wheredrawdown is zero, defines the area of influence of the well.

The shape of drawdown cones is dependent on thehydraulic parameters, which are transmissivity T andstorativity or specific yield S of the aquifers. Fig 67 showsthe various shapes for aquifers with low and high values ofT and S. Aquifers of low T values develop tight, deepdrawdown cones, whereas aquifers of high T values developshallow cones of wide extent. Transmissivity exerts a greaterinfluence on drawdown than does storativity.

If the discharge from a well and the drawdown in the welland in piezometer(s) (observation wells) placed at a knowndistance away from the pumping well are measured, we cansubstitute these into appropriate well flow equations andcalculate the hydraulic characteristics of the aquifer.

Figure 66

Drawdown in a pumped aquifer (Source: Kruseman and de Ridder, 1990)

Figure 67

Drawdown curves for aquifers with high or low transmissivity and storativity values

Module 2 – 73


Water level (drawdown) measurements

Drawdown measurements in the well and piezometersmust be measured many times during a test and with asmuch accuracy as possible. Because the water levels dropfast during the first hour or two of the test, the readings inthis period should be made at brief intervals. As pumpingcontinues, the intervals can be gradually lengthened. Table15 gives a range of intervals for water level measurementsin a pumping well and in piezometers.

Table 15 only gives a guideline on the range of intervals.The intervals should be adapted to the local conditions,available personnel, type of measuring device, etc.Nonetheless, readings in the first hours of the test shouldbe frequent, because in the analysis of the test data time isusually entered in a logarithm form. Pre-printed forms withtime, general well information and preferred time intervalsare generally used. Automatic recorders or electric dippersare used in measuring water levels.

Discharge measurements

The discharge or pumping rate should be kept constantthroughout the duration of the test and measuredperiodically. It can be kept constant by a valve in thedischarge pipe. This is a more accurate method of controlthan is changing the speed of the pump. It should however,be noted that a constant discharge rate is not requisite forthe analysis of pumping test data, as there other methodsthat take variable discharge into account.

Discharge measurements can be measured by use of a watermeter, a container of known volume, an orifice weir or anorifice bucket. The choice of the measuring device is in partdependent on the type of pump and the pumping rate.

Discharge of the pumped water should be away from thearea of influence of the pumping well in order to avoidrecycling of the water during the test.

Duration of a pumping test

The duration of a pumping test depends on the type ofaquifer and the degree of accuracy desired in establishing itshydraulic characteristics. Economizing on the period ofpumping is not recommended because the cost of runninga pump a few extra hours is low in comparison with thetotal costs of the test. It has been established that for steadystate conditions (see Section 5.4.3) to be reached, thefollowing pumping durations are required for the variousaquifers:

Type of aquifer Pumping duration

i) Leaky 15-20 hours

ii) Confined 24 hours

iii) Unconfined 3 days

It is also important to note that steady state conditions arenot requisite for a pumping duration.

5.4.2. Definition of terms

It is important for one to have a clear understanding of themeaning of common terms related with pumping tests.

Static water level (SWL):The level at which water stands in a well beforepumping takes place

Pumping water level (PWL):The level at which water stands in a well when pumpingis in progress. It is also called the dynamic water levelas it moves (declines) when the pumping rate issignificant

Drawdown:The difference, generally measured in metres, betweenthe water table or piezometric surface and the pumpingwater level (Figure 66). This difference represents thehead of water that causes the groundwater to flow intothe well

Table 15

Guideline to range of water level measurements in a pumping well and in piezometers

Pumping well Piezometer

Time from start Measuring Time from start Measuring

of pumping time intervals of pumping time intervals

0-5 minutes 0.5 minute 0-2 minutes 10 seconds

5-60 minutes 5 minutes 2-5 minutes 30 seconds

60-120 minutes 20 minutes 5-15 minutes 1 minute

120-shut down 60 minutes 15-50 minutes 5 minutes

50-100 minutes 10 minutes

100 minutes-5 hours 30 minutes

5 hours-48 hours 60 minutes

48 hours-6 days 3 times a day

6 days-shut down Once a day

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Residual drawdown:After pumping is stopped, the water level rises andapproaches the static water level observed beforepumping began. This process is known as water levelrecovery. During water level recovery, the distancebetween the water level and the initial static water levelis called the residual drawdown.

Well yield:The maximum pumping rate that can be supplied by awell without lowering the water level in the well tobelow the pump intake.

Aquifer yield:The maximum rate of withdrawal that can be sustainedby an aquifer without causing an unacceptable declinein the hydraulic head (c.f. drawdown) in the aquifer.

Specific capacity:This is a measure of the well’s productivity and is theyield per unit drawdown. It is obtained by dividing thedischarge rate of a well by the drawdown when they areboth measured at the same time.

5.4.3. Hydraulic properties of confined aquifers

In order to apply well hydraulic equations, certainassumptions and conditions have to be fulfilled. These are:

� The aquifer is confined

� The aquifer has a seemingly infinite areal extent

� The aquifer is homogeneous, isotropic and of uniformthickness over the area to be influenced by the test

� The piezometric surface is horizontal, or nearly so,prior to pumping over the area that the test willinfluence

� The aquifer is pumped at a constant discharge rate(variable discharge rates can also be sued but arebeyond the scope of this module)

� The well fully penetrates the entire aquifer thicknessand thus receives groundwater by horizontal flow

If these assumptions and conditions are met then thegroundwater, when it is pumped from a well, will flowtowards the well under (a) steady state or equilibrium conditionsor (b) unsteady state conditions.

Steady state flow

Steady state flow occurs when the change in drawdownbecomes negligibly small with time or where the hydraulicgradient has become constant, which means that drawdownvariations are zero. It should be noted, however, that truesteady state conditions are rare in a confined aquifer. The

flow equation by Theim (1906) can be used for a pumpingwell with two observation or monitoring wells orpiezometers to obtain the aquifer transmissivity T. Theequation is given by:

Equation 41

Q = 2�T(s1 - s2)

2.3log(r2 / r1)

or

T = 2.3Qlog(r2 / r1)

2�(s1 - s2)

Where:

Q = Well pumping rate (m3/d)

T = Aquifer transmissivity (m2/d)

r1 and r2 = Respective distances (r) of

piezometers (m)

s1 and s2 = Respective steady state

drawdowns in the piezometers (m)

In order to utilize this equation, the assumptions discussedat the beginning of this section for a steady state flow to thewell should be met. Theim’s method can not be used tocalculate the aquifer storativity S.

Procedure:

� Semi-log plots of either drawdown (s) versus time (t)or drawdown (s) versus distance (r) of the piezometersfrom the pumping well are constructed. Drawdown (s)is plotted on vertical axis on the linear scale and time(t) or distance (r) on the horizontal axis on alogarithmic scale.

� For a drawdown versus time plot (Figure 68), read offthe steady state drawdown si (where i = well 1, 2, 3,etc.) for each piezometer and substitute these intoEquation 40 together with the corresponding values ofr and Q and then calculate T.

The value of T can also be computed using the drawdownversus distance method, which, however, will not beexplained in this Module.

Unsteady state flow

When the flow of groundwater to a well is unsteady, whichmeans that the drawdown differences with time are notnegligible and that the hydraulic gradient is not constantwith time, Theis’s and Jacob’s methods are used. Thesemethods are not explained in detail in this Module. Theis’smethod uses matching log-log plots the measured data withthat of appropriate theoretical curves. Jacob’s method usesa similar approach as that discussed under steady state flow.

Module 2 – 75


5.4.4. Hydraulic properties of leaky and unconfinedaquifers

Methods of pumping test data analysis for steady andunsteady groundwater flow to a pumping well as well as theassumptions and conditions that have to be fulfilled whenanalyzing the data for the various aquifer types are beyondthe scope of this Module. Only general information is givenbelow and the reader is referred to more specializedliterature on the subject for further reading.

Leaky aquifers

In nature, leaky aquifers occur more frequently thanperfectly-confined aquifers (Section 5.2.2). There areassumptions and conditions that have to be fulfilled whenanalyzing data from leaky aquifers.

For proper analysis of pumping test data from a leakyaquifer, piezometers are needed in the leaky aquifer, in theaquitard and in the upper aquifer.

Figure 68

Drawdown versus time for 3 piezometers, located at 30, 90 and 215 m from a pumping well

Example 15

Application of the drawdown versus time method

Given a drawdown (s) versus time (t) semi-plot, Figure 68, for a well pumped at 788 m3/d, with 3 piezometers 30, 90and 215 m away.

What is the transmissivity T?

T =2.3 x 788log(r2 / r1)

2�(s1 - s2)

It can be noticed from Figure 68 that the curves of piezometers H30 and H90 start to run parallel approximately 10

minutes after pumping began. This means that the drawdown difference between these piezometers (s1 - s2)

remained constant from t = 10 minutes onwards, which means that the hydraulic gradient between these two

piezometers remained constant, a primary condition for which the Theim's equation is valid. The drawdown curve of

piezometer H215 does not run parallel to that of the other 2 piezometers, not at even very late pumping times. The

data for this piezometer should be neglected since it does not reflect steady state conditions.

Since (s1 - s2) remains constant from 10 minutes onwards, any time after that can be selected to determine the (s1 -

s2). Considering a t of 830 minutes, then s1 = 1.088 and s2 = 0.716 for H30 and H90 respectively �

2.3 x 788 x log(90 / 30)T =

2 x 3.14 x (1 088 - 0.716) = 370 m2 / day

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The De Glee and Hantush-Jacob methods are general usedfor steady state flow. The methods allow the characteristicsof the aquifer and aquitard to be determined.

For unsteady state flow with no aquitard storage, theWalton method and Hantush inflection point method areemployed. For cases where there is aquitard storage,Hantush’s curve fitting and the Neuman-Witherspoonmethods are used.

Unconfined aquifer

There are basic differences between confined aquifers andunconfined aquifers when they are pumped. These are:

� A confined aquifer is not dewatered during pumping (ifthe discharge rate is appropriate); it remains fullysaturated and the pumping creates a drawdown in thepiezometric surface (Figure 67). In unconfinedaquifers, the saturated thickness declines as the watertable is lowered

� Groundwater flow towards a fully penetrating well in aconfined aquifer is horizontal, unlike that inunconfined aquifers

As for other aquifers, there are assumptions and conditionsthat have to be satisfied before the well hydraulic equationsare employed for unconfined aquifers.

Well hydraulics equations for both steady state andunsteady state groundwater flow exist. For steady state flow,the Theim-Dupuit method is used to calculate the aquifertransmissivity. Neuman’s curve fitting method is used forthe calculation of unconfined aquifer hydrauliccharacteristics under unsteady state flow.

It should be noted that a number of computer packages forthe analysis of pumping test data exist on the market.However, one has to fully understand the applicability ofthe various analytical methods and not use them blindly.Moreover, the importance to collect data of good quality inthe field can not be over-emphasized.

It is also worth noting that there are other aquiferconfigurations, which require certain conditions to be metand need special analytical approach, and hence the inputof experts is critical.

5.4.5. Recovery tests

When the pump is shut down after drawdownmeasurements, the water levels in the pumped well andpiezometer(s) will start to rise. The rise in the water levelsis called residual drawdown (Section 5.4.2) and is denoteds'. It is always good practice to measure the residualdrawdowns immediately after pumping has stopped.

Recovery test measurements allow the transmissivity T ofthe aquifer to be calculated, thereby providing anindependent check on the results of drawdownmeasurements. Moreover, recovery measurements costvery little in comparison with the cost of running the pumpduring drawdown measurements.

The analysis of the recovery data is based on the principleof superposition, which means that the drawdown causedby two or more wells is the sum of the drawdown causedby each individual well. It is assumed that after the pump isshut, the well continues to be pumped at the samedischarge as before and that an imaginary well pumps waterinto the aquifer (recharge well) equal to the discharge. Thedischarge and recharge cancel each other, resulting in anideal well as required for the recovery period. Any of theflow equations discussed in the preceding sections can beformulated and employed. Theis’s recovery method is themost widely used method for the analysis of recovery testsdata.

5.4.6. Slug tests

During a slug test, a small volume of groundwater, knownas a slug, is quickly removed from a well by use of a baileror a bucket, after which the rate of rise of the water level inthe well is measured. Instead of removing the groundwaterby a bailer or bucket, a closed cylinder or other solid bodycan be submerged in the well and then, after the water levelhas stabilised, the body is pulled out. Alternatively, a smallslug of water (not contaminated and of more or less thesame quality as the groundwater) is poured into the welland the rise and subsequent fall of the water level aremeasured. From these measurements, the aquifer’stransmissivity T or hydraulic conductivity K can bedetermined.

Enough water has to be displaced or removed to raise orlower the water level by around 10-50 cm. If thetransmissivity of the aquifer is high, say 200 m2/day, thewater level will recover too quickly for accurate manualmeasurements and an automatic recorder will have to beused.

In a slug test, no pumping and piezometers are required.The test usually takes a few minutes or at the most, a fewhours. Nevertheless, slug tests are not a substitute forconventional pumping tests. It is only possible to determinethe characteristics of a small volume of aquifer materialsurrounding the well, and this volume is most likely to havebeen disturbed during drilling and construction.

For conventional slug tests carried out in confined aquiferswith fully penetrating wells, curve fitting methods havebeen developed, among others, by Cooper et al. (1967) and

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Papadopulos and Cooper (1973). For wells partially or fullypenetrating unconfined aquifers, the Bouwer and Rice(1976) method can be used.

5.4.7. Well performance tests

A well performance test is conducted to determine lossesattributed to the aquifer and the well. Aquifer losses arehead losses that occur in the aquifer where the flow islaminar. These are time-dependent and vary linearly withdischarge.

Well losses are divided into linear and non-linear headlosses (Figure 69). Linear well losses are caused by‘damaging’ or disturbing the aquifer material during drillingand they comprise, for example, head losses in the gravelpack (Section 5.3.2) and in the screen. Non-linear welllosses are the friction losses that occur inside the wellscreen and in the suction pipe where the flow is turbulent.All these losses are responsible for the drawdown inside thewell being much greater than would have been under idealconditions. Step drawdown tests are used to determinethese losses.

Step drawdown test

A step drawdown test is a test in a single well in which thewell is pumped at a low constant discharge rate until thedrawdown within the well stabilizes. The pumping rate isthen increased and pumping continued until the drawdownstabilizes once more. The process is repeated at least threetimes, each of which should be of equal duration of say anhour each. The collected data are used in the computationof aquifer and well losses.

The following equation is commonly used:

Equation 42

sw = BQ + CQ2

Where:

sw = Drawdown in a pumped well

B = Linear losses (assumed to be constant, see

below)

C = Non-linear losses (a constant)

Q = Discharge rate

Knowing B and C, we can predict the drawdown in a wellfor any realistic discharge Q. B is time-dependent, but sinceit changes only slightly after a reasonable pumping duration,it can thus be assumed to be constant.

Figure 69

Various head losses in a pumped well (Source: Kruseman and de Ridder, 1970)

Specific capacity

Specific capacity Sc has been defined earlier (Section 5.4.2)as a measure of a well’s productivity and is obtained by:

Equation 43

Sc = Q/sw

Where:

Sc = Specific capacity

Q = Discharge rate

Sw = Drawdown measured in the field

The larger the specific capacity, the better the well.Observing the change in drawdown and specific capacitywith increased discharge provides information required toselect optimum pumping rates or to obtain information onthe condition or efficiency of the well.

Well efficiency

Well efficiency E, expressed as a percentage, is the ratio ofthe theoretical drawdown s1 to the drawdown measured inthe field sw (Figure 69) or the ratio of the field specificcapacity Q/sw to the theoretical specific capacity for aspecific duration of pumping. It is given by the expression:

Equation 44

E = Q /sw x 100

=s1 x 100

Q / s1 sw

Where:

E = Well efficiency (%)

Q = Discharge rate

s1 = Theoretical drawdown

sw = Drawdown measured in the field

E values close to 100% indicate an efficient well withminimal well losses. Low E values, say 40%, are indicativeof poorly-constructed and poorly developed wells.

Inefficient wells can also be recognized by noting the initialrecovery rate when pumping is stopped. Where the wellloss is large, the drawdown recovers rapidly by drainage intothe well from the surrounding aquifer. A rough rule ofthumb for this purpose is:

If a pump is shut off after 1 hour of pumping and 90% or more ofthe drawdown is recovered after 5 minutes, the well can be consideredunacceptably inefficient.

Well inefficiency is caused by the following factors, whichcan be grouped into two classes:

i) Design factors:

– The choice of screens with insufficient open arearesults in high entrance velocities causing higherthan normal entrance (head losses).

– Poor distribution of screen openings causesexcessive convergence of flow near the individualopenings, and may produce twice as muchdrawdown as necessary.

– Partial penetration of screens into aquifers distortsthe flow around the well. Flow to the well willinclude both the horizontal and verticalcomponents. If the vertical conductivity is greaterthan the horizontal one, considerable head lossesresult from the vertical flow.

– A wrong choice of filter material or gravel pack, forexample angular grains or plate-like material, hasbeen made.

ii) Construction factors:– Inadequate development of the well: fine material

or drilling mud reduces original permeability– Improper screening: placing of screens on non-

water-bearing stratum

Well interference

Well interference occurs when the cones of depression(Section 5.4.1) of two or more wells overlap. The spacingbetween wells determines the amount of interference. Ingeneral, wells in a well field designed for huge water supplies,for example for irrigation or municipal purposes, should bespaced so as to minimize the effects of interference, whichwould otherwise result in drastic lowering of water levels.Well interference is sometimes encouraged to lowergroundwater levels and thus create huge storage forgroundwater recharge (Section 5.5) during rainy seasons thatcan be used during the dry periods. Well interference canalso be used to control water table elevations.

Advantages and disadvantages of pumping tests

Advantages of carrying out pumping tests are:

� A pumping test provides in situ parameter values thatare representative of the aquifer

� Information on a number of hydraulic parameters suchas transmissivity, hydraulic conductivity, storativity,specific yield, leakance, hydraulic diffusivity, etc. can beobtained from a single test, unlike laboratory tests thatare based on sample measurements which, in mostcases, will not be representative of the aquifer material.It is not possible to obtain all the hydraulic parametersfrom laboratory measurements. The same can be saidof tracer techniques

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� It is possible to predict the effect of new withdrawals(pumping) on existing wells

� The drawdown in a well at future times and differentdischarges can be predicted

� The radius of influence of a single well or a multiple ofwells can be determined before the well or wells are sunk

� It provides very crucial information needed to conductan effective and efficient groundwater managementprogramme

One scientific and one practical disadvantage of pumpingtests can be mentioned:

� The scientific limitation relates to the non-uniquenessof pumping test interpretation. In the absence of cleargeological information, leaky, unconfined and boundedaquifers can give similar time-drawdown responses. Insuch cases, the calculated hydraulic parameters will bemisleading

� The practical limitation lies in the cost of conductingpumping tests. Pumping wells and observation wells orpiezometers will have to be drilled so as to obtain someof the aquifer hydraulic properties. It is worth notingthat for aquifers intended for supply of irrigation water,the cost of such tests are outweighed by the overallirrigation investment. Risks for irrigation projectscollapsing midway due to lack of proper well designand the computation of optimum pumping rates (frompumping test analysis) exist if the hydraulic propertiesof the aquifer are not known

5.5. Groundwater recharge

Groundwater recharge, expressed in mm/year, can bedefined as the entry of water into a groundwater body oraquifer after infiltration and percolation through theunsaturated zone. Principal sources of natural rechargeinclude precipitation (which is mostly rainfall in East andSouthern Africa), rivers and reservoirs. Other sources ofrecharge are irrigation return flows and seepage fromcanals. The amount of annual recharge is very importantsince it is what defines the sustainable yields of aquifers. Ifgroundwater abstractions greatly exceed the amount ofrecharge, groundwater mining results.

There are a number of techniques used in the estimation ofgroundwater recharge. Some of them are described below.

5.5.1. Water balance equations

Simple water budget equations can be used in theestimation of recharge, and an example in the recharge areaof a catchment is:

Equation 45

P = RO + RG + ET

Where:

P = Average annual precipitation (rainfall in our

case)

RO = Average annual surface runoff

RG = Average annual groundwater recharge

ET = Average annual evapotranspiration

The amount of recharge can thus be estimated if all theother parameters are accurately measured or estimated.The average annual rainfall and runoff can be estimatedwith some degree of accuracy. The accurate determinationof evapotranspiration is difficult, since on a catchment scaleit can only be assessed by indirect measurements andthrough the use of formulae such as the Penman-Monteithmethod.

5.5.2. Chloride mass balance technique

This is an inexpensive and relatively simple technique ofestimating groundwater recharge. Although the techniqueis not as accurate as other methods, differences in rechargeestimation are still within the order of magnitude. Thetechnique is based on the assumption of conservation ofmass balance between the input of atmospheric chlorideand the chloride flux in the subsurface (again assuming thatthere are no sources or sinks for chloride) and under steadystate conditions. In that case, the following mass balanceequation can be defined:

Equation 46

RT x Clgw = P x Clp + D

Where:

RT = Average total recharge

Clgw = Average chloride concentration in

groundwater

P = Average rainfall

Clp = Average chloride concentration in

rainfall

D = Average dry chloride deposition

measured during the dry periods

As all parameters except RT can be measured, the averagetotal recharge RT can then be computed from:

RT =P x CLp + D

CLgw

The method has been extensively employed in groundwaterrecharge and groundwater resources assessment of theKalahari in Botswana. Usage of the technique in Zimbabweis still in its early stages.

Module 2 – 79


5.5.3. Groundwater level fluctuations method

Groundwater level fluctuations reflect the response ofunconfined aquifer systems to recharge, pumping andnatural losses such as evapotranspiration and baseflow(groundwater flow to rivers or streams). The rise in waterlevel (�h) at a particular location after a rainfall event canbe converted to recharge by using the following formula:

Equation 47

R = �h x Sy

Where:

R = Groundwater recharge

�h = Rise in water level

Sy = Specific yield (Section 5.4.2), which can be

obtained from the analysis of pumping tests

data

The accuracy of R strongly depends on the specific yield.The estimation of groundwater recharge using thistechnique has been carried out in the NyamandhlovuSandstone Aquifer in Zimbabwe, the Wondergat dolomitesink hole in South Africa and in the Kalahari in Botswana.

5.5.4. Environmental isotopes

Environmental isotopes commonly used in groundwaterresources investigation are oxygen 18 and hydrogen 2(known as deuterium), radiocarbon (carbon 14) andtritium. Oxygen 18 and hydrogen 2 are stable isotopes andare used, inter alia, to determine the origin or source of thegroundwater, sources of groundwater salinity and mixingbetween various water bodies. Radiocarbon and tritium areused to determine the ‘age’ of groundwater, which will beindicative of whether the groundwater is being activelyrecharged or whether it was recharged some time ago.

5.6. Groundwater management

Groundwater management constitutes the developmentand sustainable utilization of groundwater resourceswithout compromising its quantity and quality. Effectiveand efficient groundwater management requires theestablishment of a monitoring programme, which looks at(i) groundwater level fluctuations, (ii) abstraction rates, and(iii) water quality aspects. Groundwater models are anessential management tool.

5.6.1. Groundwater models

Models are important as predictive tools to estimate howthe groundwater quality or quantity may change givencertain conditions such as variation in pumping rates,drawdowns or groundwater levels, recharge rates, etc. It is

also possible to predict water levels given pumping rates orvice versa. The effects of boundary conditions, such assurface water (rivers and reservoirs) contribution togroundwater, can also be assessed. Two general types ofnumerical models exist, the finite difference and finiteelement models. Modflow is a commercially available finitedifference computer model.

5.6.2. Lowering groundwater levels

Aquifers adjoining rivers or other surface water sources orwith rivers running through them can potentially berecharged from the surface water. This can be establishedby the use of isotopes (Section 5.5.4). In such cases, areduction in the groundwater levels induces recharge fromthe surface water source, if the surface water body ishydraulically linked to the aquifer. It is good managementpractice to draw down the groundwater table during dryperiods. The reduction in the water table would betemporary and rapid recovery could be expected duringnormal rainy seasons.

5.6.3. Conjunctive use of surface water andgroundwater

Conjunctive use involves the coordinated and plannedutilization of both surface water and groundwater resourcesto meet water requirements in a manner where water isconserved. In a conjunctive scheme, during periods ofabove normal rainfall surface water is utilized to themaximum extent possible and, where feasible, artificiallyrecharged (pumped into aquifers through wells known asinjection wells) into the aquifer to augment groundwaterstorage and raise groundwater levels (care should be takennot the raise the levels to the crop root zone). Conversely,during drought periods the limited surface water resourceswill be supplemented by pumping groundwater, therebylowering the water levels. However, the cost of setting upsuch a scheme could be prohibitive for most Africancountries.

5.6.4. Groundwater monitoring

Groundwater monitoring is a prerequisite for optimalgroundwater resource management so as to achievesustainable use in terms of both quantity and quality overthe long term. A monitoring programme routinely providesa continuous record of the aquifer’s response to variousinputs and outputs including recharge, evapotranspiration,baseflow and abstraction, as manifested by changes in waterlevels and water quality with time. Knowledge of theresponse of the aquifer to these factors with time thereforeenables groundwater resources management to beconducted efficiently and effectively.

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The routine monitoring of groundwater abstractions,groundwater level fluctuations and water quality data iscrucial. Any variations in these can, with time, provide earlywarning signs of the deterioration in groundwater quantityor quality and will allow for early remedial action to betaken. A very good electronic hydrogeological database,linked to a surface water one where feasible, is a must sincethis will make data manipulation and analysis a lot easierand much quicker. Time series plots of importantparameters would be easy to generate. Moreover, the dataare very important in refining groundwater models (Section5.6.1) and thus assist in the accurate predictions of variousscenarios for current groundwater development and futuregroundwater needs.

Figure 70 shows a hydrograph (groundwater levelfluctuation with time) of a well in the NyamandhlovuSandstone Aquifer in southwestern Zimbabwe. The

hydrograph shows a continuous groundwater level decline,about 0.25 m/year, with little or no indication ofgroundwater levels recovery. The decline is a result of highabstraction or discharge rate and sooner or later thegroundwater level would be drawn down to the pumpintake.

If there were saline groundwater below the fresh water,saline water upconing would be inevitable. Moreover,lowered drawdowns translate to increased pumping costs.It is thus apparent that it is not good groundwatermanagement practice to cause severe drawdowns in semi-arid regions where recharge is limited. Pumping heavilyfrom these aquifers provides only a short-term solution fora long-term problem. Too often, short-term economicconsiderations become the primary factor affectinggroundwater management.

Figure 70

Hydrograph showing continuous groundwater level decline (the well is from Nyamandhlovu Aquifer in

south western Zimbabwe)

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Abderraham, W. A. et al. 1995. Impacts of management practices on groundwater conditions in the Eastern Province, SaudiArabia. Hydrogeology Journal, Volume 3 No. 4, 32-41 pp.

Barnister, A. and Raymond, S. 1986. Surveying. Fifth edition, English Language Book Society. Longman, London.

Beekman, H. E. et al. 1997. Chloride and isotope tracer profiling studies in the Letlhakeng-Botlhapatlou area and the central Kalahari.GRESS II Technical Report: Groundwater recharge and resource assessment in the Botswana Kalahari.

Bouwer, H. 1978. Groundwater hydrology.

Bouwer, H. and Rice, C.H. 1976. A slug test method for determining hydraulic conductivity of unconfined aquifers withcompletely or partically penetrating wells. Water Resources Research, Volume 12, No. 3.

Bredenkamp, B.D., Botha, L., J., van Tonder, G. L. and van Rensburg, H. J. 1995. Manual on quantitative estimation ofgroundwater recharge and aquifer storativity.

Byron, J. 1994. Spectral encoding of soil texture: a new vizualization method. Department of Geography, University of SouthCarolina, USA. GIS/LIS.

Chalingar, G.V. 1963. Relationship between porosity, permeability, and grain size distribution of sands and sandstones.Proceedings of the International Conference on Sedimentology, Amsterdam, Antwerp.

Chettri, M. and Smith, G.D. 1995. Nitrate pollution in groundwater in selected districts of Nepal. Hydrogeology Journal,Volume 3 No. 1, 71-76 pp.

Clarke, I., and Fritz, P. 1997. Environmental isotopes in hydrogeology.

Cooper, H.H., Bredehoeft and Papadopulos, I.S. 1967. Response of a finite-diameter well to an instantaneous recharge ofwater.

Davis, S.N. 1969. Porosity and permeability of natural materials. Flow through porous media, edited by R.J.M. De Wiest,Academic Press, New York, 54-89 pp.

Di Gregorio, A. and Jansen, L.J.M. 1998. Land cover classification system (LCCS): Classification concepts and user manual. Forsoftware version 1.0. GCP/RAF/287/ITA Africover – East Africa Project in cooperation with AGLS and SDRN.Nairobi, Rome.

Driscoll, F.G. 1986. Groundwater and wells. Second edition.

Eijkelkamp. Undated. Training equipment for schools.

Euroconsult. 1989. Agricultural compendium for rural development in the tropics and subtropics. Elsevier, Amsterdam, TheNetherlands.

FAO. 1979. Soil survey investigations for irrigation. FAO Soils Bulletin 42. FAO, Rome, Italy.

FAO. 1985a. Guidelines: Land evaluation for irrigated agriculture. FAO Soils Bulletin 55. FAO, Rome, Italy.

FAO. 1985b. Elements of topographic surveying. Prepared by C. Brouwer, A. Goffeau, J. Plusjé and M. Heibloem. Irrigationwater management training manual No 2. Rome, Italy.

References

Irrigation manual

84 – Module 2

FAO. 1985c. Water quality for agriculture. FAO Irrigation and Drainage Paper Number 29. Prepared by: Ayers, R.S. and Westcot,D. W. Rome.

FAO/UNEP. 1997. Negotiating a sustainable future for land. Structural and institutional guidelines for land resources management in the21st Century. FAO/UNEP, Rome, Italy.

FAO/UNEP. 1999. The future of our land: facing the challenge. Guidelines for integrated planning for sustainable management of landresources. FAO/UNEP, Rome, Italy.

FitzPatrick, E.A. 1980. Soils: their formation, classification and distribution. Longman Publishers, New York.

Foster, S.S.D. and Smith-Carrington, A. 1980. The interpretation of tritium in the chalk unsaturated zone. Journal ofHydrology. Volume 46, 343-364 pp.

Freeze, R.A. and Cherry, J. A. 1979. Groundwater.

Geological Survey of Germany. 1996. Limestone and dolomite resources of Africa.

Government of Zimbabwe. 1998. Zimbabwe Water Act 1998.

International Atomic Energy Agency. 1981. Stable isotope hydrology, deuterium and oxygen-18 in the water cycle. TechnicalReports Series No. 210. Edited by J.R. Gat and R. Gonfiantini.

International Hydrological Programme. 2000. Environmental isotopes in the hydrological cycle: Principles and applications;Volume IV Number 39 Groundwater - saturated and unsaturated zone. Edited by W.G. Mook.

International Hydrological Programme. 2001. Environmental isotopes in the hydrological cycle: Principles and applications; Volume VNo.39 Man’s impact on groundwater systems. Edited by W.G. Mook.

Jensen, M.E. 1983. Design and operation of farm irrigation systems. American Society of Agricultural Engineers (ASAE).

Kamp, G. van der. 2001. Methods of determining the in situ hydraulic conductivity of shallow aquitards - an overview.Hydrogeology Journal, Volume 9 No. 1, 5-16 pp.

Kern & Co. Ltd. Undated. Instruction manual for Kern GK 0 Builder’s level and Kern GK 1 Small Engineer’s level. Switzerland.

Kruseman, G.P. and de Ridder, N. A. 1990. Analysis and evaluation of pumping test data. Second edition (completely revised).

Leica, Ag. 1993. Level accessories, Product catalogue. Heerbrugg, Switzerland.

Leica Geosystems. Undated. Surveying made easier. By: K. Zeiske.

Mather, J.D. 1975. Development of groundwater resources of small limestone islands. Quarterly Journal of Engineering Geology,Volume 8, 141-150 pp.

MEWRD (Ministry of Energy, Water Resources and Development). 1984. An assessment of the surface water resources of Zimbabweand guidelines for development planning. Zimbabwe.

MEWRD (Ministry of Energy, Water Resources and Development) 1988. Sedimentation and yields of small dams. Zimbabwe.

Ministère de la coopération française. 1980. Memento de l’agronome.

Papadopulos, I.S. and Cooper, H.H. 1973. Drawdown in a well of large diameter. Water Resources Research, Volume 3.

Terao, H., Yoshika Y. and Kato, K. 1993. Groundwater pollution by nitrate originating from fertiliser in KakamigaharaHeights, central Japan. International Association of Hydrogeologists, Volume 4.

Theim, G. 1906. Hydrologische methoden. Leipzig, 56 p.

Thompson, J.G. and Purves, W.D. 1979. The Assessment of the suitability of soils for irrigation. Rhodesia Agricultural JournalVol 76 (3).

Module 2 – 85


Ting, C.S., Kerh, T., and Liao, C. J. 1998. Estimation of groundwater recharge using the chloride mass balance method,Pingtung Plain, Taiwan. Hydrogeology Journal, Volume 6 No. 2, 282-292 pp.

Todd, D.K. 1976. Groundwater hydrology.

Tucker, M.E. 1991. An introduction to the origin of sedimentary rocks.

Uren, J and Price, W. F. 1985. Surveying for engineers. Second edition.

Ward, R.C. and Robinson M. 1990. Principles of hydrology. Third edition.

Wild Heerbrugg Ltd. Undated. Levelling, Users’ manual. Switzerland.

Wild Heerbrugg Ltd. Undated. The theodolite and its applications, Users’ manual. Switzerland

Wilson, R.J.R. 1971. Land Surveying.

Winter, T.C. 1999. Relation of streams, lakes, and wetland to groundwater flow systems. Hydrogeology Journal, Volume 7,No.1, 28-45 pp.

Withers, B. and Vipond, S. 1974. Irrigation: Design and practice. Batsford, London.

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